[backport#16452] refactor : use RelayTransaction in BroadcastTransaction utility

Summary:
To do so, we also refactor RelayTransaction to take a txid
instead of passing a tx

https://github.com/bitcoin/bitcoin/pull/16452/commits/9bc8b28c1d26c28edf4bbc890be97c0ad7a73cb9

Backport of Core PR16452

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6272