feat(admin): support service account updates

[cxymds]

Related issue

Closes #261

Background

The CLI supports creating, inspecting, listing, and removing service accounts, but it cannot update an existing account even though the RustFS Admin API exposes the operation. Infrastructure automation therefore has to recreate accounts just to change a policy or another mutable field.

Root cause

The core admin abstraction and S3 admin client did not expose the update-service-account endpoint, and the CLI had no command for constructing partial update requests.

Solution

• Add a partial service account update request to the core admin API.
• Call POST /rustfs/admin/v3/update-service-account?accessKey=... from the S3 admin client.
• Add rc admin service-account update with edit and set aliases.
• Support policy, secret key, status, name, description, and expiration updates while preserving unspecified fields.
• Reject empty updates and unreadable policy files with usage errors.
• Keep replacement secret keys out of command output.
• Add unit, HTTP, CLI integration, and help-contract coverage plus a README example.

User impact

Service account permissions and metadata can now be managed declaratively without deleting and recreating credentials.

Validation

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