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

1 files changed, 32 insertions, 0 deletions

diff --git a/weed/iam/constants.go b/weed/iam/constants.go
new file mode 100644
index 000000000..0b857a896
--- /dev/null
+++ b/weed/iam/constants.go
@@ -0,0 +1,32 @@
+package iam
+
+// Character sets for credential generation
+const (
+	CharsetUpper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
+	Charset      = CharsetUpper + "abcdefghijklmnopqrstuvwxyz/"
+)
+
+// Policy document version
+const PolicyDocumentVersion = "2012-10-17"
+
+// Error message templates
+const UserDoesNotExist = "the user with name %s cannot be found."
+
+// Statement action constants - these map to IAM policy actions
+const (
+	StatementActionAdmin    = "*"
+	StatementActionWrite    = "Put*"
+	StatementActionWriteAcp = "PutBucketAcl"
+	StatementActionRead     = "Get*"
+	StatementActionReadAcp  = "GetBucketAcl"
+	StatementActionList     = "List*"
+	StatementActionTagging  = "Tagging*"
+	StatementActionDelete   = "DeleteBucket*"
+)
+
+// Access key lengths
+const (
+	AccessKeyIdLength     = 21
+	SecretAccessKeyLength = 42
+)
+