feat(admin): add access key info command

[cxymds]

Summary

Add a general admin access-key inspection command:

rc admin access-key info <ALIAS> <ACCESS_KEY>

The command calls RustFS Admin API GET /v3/info-access-key and reports whether the key belongs to a regular user, service account, or STS credential. It supports both human-readable and JSON output.

Changes

• Add AccessKeyInfo response types to rc_core::admin.
• Add AdminApi::get_access_key_info and implement it in rc_s3::AdminClient.
• Add rc admin access-key info CLI command.
• Preserve RustFS response field names such as openIDSpecificInfo, userID, and userIDClaim.
• Map access key not exist for this command to a NotFound exit result.
• Document the command in README.md.

Verification

• cargo fmt --all --check
• cargo clippy --workspace -- -D warnings
• cargo test --workspace

Risks

• No breaking changes to existing commands.
• No protected docs or JSON schema files were modified.
• The command does not print secret keys or session tokens.

[cxymds]

rustfs/rustfs#3281