feat(funding-service): add the RequestFunds endpoint

[SantiagoPittella]

Summary

Add the RequestFunds endpoint. Each request creates a private pay-to-ID note for the requested account, and the service answers once that note is committed in a block, returning the note together with proof of its inclusion.

One worker owns the funding account. Requests that arrive while a transaction is in flight share the next one, up to --max-notes-per-tx, so a client topping up several accounts waits for one transaction rather than one per account. Errors carry a one-byte code in the gRPC status details.

Changelog

[[entry]]
scope       = "funding-service"
impact      = "added"
description = "`RequestFunds` sends the native asset to an account as a private pay-to-ID note and returns it once committed."