feat(node): prefund registered accounts

[Mirko-von-Leipzig]

Summary

• Add --funding-service.url and --funding-service.amount to the sequencer. Both options must be set together. The amount is a positive number of native-asset base units.
• Request funding inline after each new account registration through RegisterAccount or the admin API, including imports that bind an existing invitation.
• Use the POST /request-funds JSON API from feat(funding-service): add the RequestFunds endpoint #2601. This PR assumes that service is deployed; it does not include the service implementation.

Registration commits before the funding request. The call waits for the service to confirm the public P2ID note. Funding failures return gRPC UNAVAILABLE or HTTP 503, without undoing registration. Identical registration retries do not request more funds. There is no background worker, persistent queue, or automatic retry. Account owners retrieve funding notes through their account's note tag.

Changelog

[[entry]]
scope = "node"
impact = "added"
description = "Sequencers can prefund newly registered accounts through a configured funding service."

[kkovaacs]

Looks good to me!