feat: add admin commands for IAM user, policy, group, and service account management

Implement CLI support for IAM management on RustFS/MinIO-compatible servers.
This adds the 'rc admin' command with subcommands for managing:

- Users: list, add, info, remove, enable, disable
- Policies: list, create, info, remove, attach
- Groups: list, add, info, remove, enable, disable, add/remove members
- Service Accounts: list, create, info, remove

New modules:
- core/admin: AdminApi trait and IAM types (User, Group, Policy, ServiceAccount)
- s3/admin: AdminClient implementing HTTP calls with AWS SigV4 signing
- cli/commands/admin: CLI command implementations

All commands support --json output for scripting.

Closes #1571

Author: overtrue

Cargo.lock

@@ -60,6 +60,14 @@ async-trait = "0.1"
 mime_guess = "2.0"
 glob = "0.3"
 
+# HTTP client for Admin API
+reqwest = { version = "0.12", default-features = false, features = ["rustls-tls", "json"] }
+aws-sigv4 = "1.2"
+http = "1.2"
+sha2 = "0.10"
+hex = "0.4"
+urlencoding = "2.1"
+
 # Testing
 tempfile = "3.24"
 mockall = "0.14"