Add policy engine (#6970)

weed/s3api/policy_engine/POLICY_EXAMPLES.md

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+# Policy Engine Examples
+
+This document contains examples of how to use the SeaweedFS Policy Engine.
+
+## Overview
+
+The examples in `examples.go` demonstrate various policy configurations and usage patterns. The examples file is excluded from production builds using build tags to reduce binary size.
+
+## To Use Examples
+
+If you need to use the examples during development or testing, you can:
+
+1. **Remove the build tag**: Remove the `//go:build ignore` and `// +build ignore` lines from `examples.go`
+2. **Use during development**: The examples are available during development but not in production builds
+3. **Copy specific examples**: Copy the JSON examples you need into your own code
+
+## Example Categories
+
+The examples file includes:
+
+- **Legacy Identity Format**: Examples of existing identities.json format
+- **Policy Documents**: Various AWS S3-compatible policy examples
+- **Condition Examples**: Complex condition-based policies
+- **Migration Examples**: How to migrate from legacy to policy-based IAM
+- **Integration Examples**: How to integrate with existing systems
+
+## Usage Functions
+
+The examples file provides helper functions:
+
+- `GetAllExamples()`: Returns all example policies
+- `ValidateExamplePolicies()`: Validates all examples
+- `GetExamplePolicy(name)`: Gets a specific example
+- `CreateExamplePolicyDocument(name)`: Creates a policy document
+- `PrintExamplePolicyPretty(name)`: Pretty-prints an example
+- `ExampleUsage()`: Shows basic usage patterns
+- `ExampleLegacyIntegration()`: Shows legacy integration
+- `ExampleConditions()`: Shows condition usage
+- `ExampleMigrationStrategy()`: Shows migration approach
+
+## To Enable Examples in Development
+
+```go
+// Remove build tags from examples.go, then:
+import "github.com/seaweedfs/seaweedfs/weed/s3api/policy_engine"
+
+// Use examples
+examples := policy_engine.GetAllExamples()
+policy, err := policy_engine.GetExamplePolicy("read-only-user")
+```
+
+## Note
+
+The examples are excluded from production builds to keep binary size minimal. They are available for development and testing purposes only.