Add credential storage (#6938)

* add credential store interface

* load credential.toml

* lint

* create credentialManager with explicit store type

* add type name

* InitializeCredentialManager

* remove unused functions

* fix missing import

* fix import

* fix nil configuration

weed/command/scaffold/example.go

@@ -19,3 +19,6 @@ var Master string
 
 //go:embed shell.toml
 var Shell string
+
+//go:embed credential.toml
+var Credential string