Refactor: Extract common IAM logic into shared weed/iam package (#7747)

This resolves GitHub issue #7747 by extracting duplicated IAM code into
a shared package that both the embedded S3 IAM and standalone IAM use.

New shared package (weed/iam/):
- constants.go: Common constants (charsets, action strings, error messages)
- helpers.go: Shared helper functions (Hash, GenerateRandomString,
  GenerateAccessKeyId, GenerateSecretAccessKey, StringSlicesEqual,
  MapToStatementAction, MapToIdentitiesAction, MaskAccessKey)
- responses.go: Common IAM response structs (CommonResponse, ListUsersResponse,
  CreateUserResponse, etc.)
- helpers_test.go: Unit tests for shared helpers

Updated files:
- weed/s3api/s3api_embedded_iam.go: Use type aliases and function wrappers
  to the shared package, removing ~200 lines of duplicated code
- weed/iamapi/iamapi_management_handlers.go: Use shared package for constants
  and helper functions, removing ~100 lines of duplicated code
- weed/iamapi/iamapi_response.go: Re-export types from shared package for
  backwards compatibility

Benefits:
- Single source of truth for IAM constants and helpers
- Easier maintenance - changes only need to be made in one place
- Reduced risk of inconsistencies between embedded and standalone IAM
- Better test coverage through shared test suite

weed/iam/helpers.go

@@ -0,0 +1,126 @@
+package iam
+
+import (
+	"crypto/rand"
+	"crypto/sha1"
+	"fmt"
+	"math/big"
+	"sort"
+
+	"github.com/seaweedfs/seaweedfs/weed/s3api/s3_constants"
+)
+
+// Hash computes a SHA1 hash of the input string.
+func Hash(s *string) string {
+	h := sha1.New()
+	h.Write([]byte(*s))
+	return fmt.Sprintf("%x", h.Sum(nil))
+}
+
+// GenerateRandomString generates a cryptographically secure random string.
+// Uses crypto/rand for security-sensitive credential generation.
+func GenerateRandomString(length int, charset string) (string, error) {
+	if length <= 0 {
+		return "", fmt.Errorf("length must be positive, got %d", length)
+	}
+	if charset == "" {
+		return "", fmt.Errorf("charset must not be empty")
+	}
+	b := make([]byte, length)
+	for i := range b {
+		n, err := rand.Int(rand.Reader, big.NewInt(int64(len(charset))))
+		if err != nil {
+			return "", fmt.Errorf("failed to generate random index: %w", err)
+		}
+		b[i] = charset[n.Int64()]
+	}
+	return string(b), nil
+}
+
+// GenerateAccessKeyId generates a new access key ID.
+func GenerateAccessKeyId() (string, error) {
+	return GenerateRandomString(AccessKeyIdLength, CharsetUpper)
+}
+
+// GenerateSecretAccessKey generates a new secret access key.
+func GenerateSecretAccessKey() (string, error) {
+	return GenerateRandomString(SecretAccessKeyLength, Charset)
+}
+
+// StringSlicesEqual compares two string slices for equality, ignoring order.
+// This is used instead of reflect.DeepEqual to avoid order-dependent comparisons.
+func StringSlicesEqual(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	// Make copies to avoid modifying the originals
+	aCopy := make([]string, len(a))
+	bCopy := make([]string, len(b))
+	copy(aCopy, a)
+	copy(bCopy, b)
+	sort.Strings(aCopy)
+	sort.Strings(bCopy)
+	for i := range aCopy {
+		if aCopy[i] != bCopy[i] {
+			return false
+		}
+	}
+	return true
+}
+
+// MapToStatementAction converts a policy statement action to an S3 action constant.
+func MapToStatementAction(action string) string {
+	switch action {
+	case StatementActionAdmin:
+		return s3_constants.ACTION_ADMIN
+	case StatementActionWrite:
+		return s3_constants.ACTION_WRITE
+	case StatementActionWriteAcp:
+		return s3_constants.ACTION_WRITE_ACP
+	case StatementActionRead:
+		return s3_constants.ACTION_READ
+	case StatementActionReadAcp:
+		return s3_constants.ACTION_READ_ACP
+	case StatementActionList:
+		return s3_constants.ACTION_LIST
+	case StatementActionTagging:
+		return s3_constants.ACTION_TAGGING
+	case StatementActionDelete:
+		return s3_constants.ACTION_DELETE_BUCKET
+	default:
+		return ""
+	}
+}
+
+// MapToIdentitiesAction converts an S3 action constant to a policy statement action.
+func MapToIdentitiesAction(action string) string {
+	switch action {
+	case s3_constants.ACTION_ADMIN:
+		return StatementActionAdmin
+	case s3_constants.ACTION_WRITE:
+		return StatementActionWrite
+	case s3_constants.ACTION_WRITE_ACP:
+		return StatementActionWriteAcp
+	case s3_constants.ACTION_READ:
+		return StatementActionRead
+	case s3_constants.ACTION_READ_ACP:
+		return StatementActionReadAcp
+	case s3_constants.ACTION_LIST:
+		return StatementActionList
+	case s3_constants.ACTION_TAGGING:
+		return StatementActionTagging
+	case s3_constants.ACTION_DELETE_BUCKET:
+		return StatementActionDelete
+	default:
+		return ""
+	}
+}
+
+// MaskAccessKey masks an access key for logging, showing only the first 4 characters.
+func MaskAccessKey(accessKeyId string) string {
+	if len(accessKeyId) > 4 {
+		return accessKeyId[:4] + "***"
+	}
+	return accessKeyId
+}
+