TW-4827: add stored signatures support

[qasim-nylas]

Summary

• add stored signatures support to the CLI, including signature CRUD commands and signature_id support for send and draft flows
• harden draft sending so --signature-id cannot stack or duplicate an already-managed stored signature
• disable automatic retries for non-idempotent signature create/update requests to avoid replayed writes under retriable failures

What changed

• added a new signatures adapter with demo/mock implementations and CLI commands for list/show/create/update/delete
• wired signature_id through message send, draft create/send, and related request/domain types
• added validation and support helpers around stored signatures, including protection for unsupported combinations
• updated docs for the new email signatures commands and usage

Safety and behavior

• CreateSignature and UpdateSignature no longer inherit the generic retry path used for idempotent requests
• draft send now checks for an already-managed stored signature before allowing another --signature-id to be applied
• coverage includes unit tests, adapter tests, CLI tests, and integration tests for signatures lifecycle and edge cases

Testing

• make ci-full

Jira

• Ticket: TW-4827
• Epic: TW-4638

[AaronDDM]

LGTM 👍