main: Merge signatures when re-signing a partially signed transaction
main: Merge signatures when re-signing a partially signed transaction

Summary: There was a problem with the signAztecTransaction call in the RPC API where partial signatures would not be merged together, passing along the partial signature to the signing function corrects this issue.

Reviewers: ben, johann

Reviewed By: johann

Differential Revision: https://phabricator.bitgo.com/D4155

BitGo/prova,BitGo/prova