Relayer integration for transaction submission

[aristidesstaffieri]

Part of the Improve Agent UX for Wallets epic.

Summary

Add a relayer service integration to the transaction submission flow. Like RPC, users can configure a relayer endpoint that wraps transactions in a fee-bump transaction and passes them to the relayer service for signing and submission.

Scope

• Configure via --relayer-url / STELLAR_RELAYER_URL and --relayer-header flags.
• Represent config as Option<Relayer>; transactions are self-funded in its absence (behavior unchanged when not configured).
• Add a new external-signer kind representing the relayer external signer (extend cmd/soroban-cli/src/signer/).
• The relayer owns transaction signing and submission (via a CAP-15 fee-bump envelope); the CLI polls the relayer for inclusion status.
• Expose the transaction ID in the intermediary response to allow alternate status polling.
• Hook into the cmd/soroban-cli/src/tx.rs submission flow and existing fee-bump support (assembled.rs).

Acceptance criteria

• With a relayer configured, a transfer is wrapped in a fee bump, handed to the relayer, and polled to inclusion.
• The intermediary response exposes the transaction ID for alternate status polling.
• Without a relayer configured, submission behavior is unchanged (self-funded).

Dependencies

Largely independent; plugs into the tx.rs submission flow.