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

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