feat(bridge-withdrawer): support FROST threshold signing

[noot]

Summary

support FROST ed25519 threshold signing in the bridge withdrawer.

Background

improves security of the withdrawer over having one private key used; instead, we can employ an m-of-n threshold signature scheme to sign withdrawals.

Changes

• create FrostParticipantService gRPC proto definition, which is to be implemented by a binary which contains a FROST secret key partial that participates in the threshold signing process
• update withdrawer config/setup to support single signer (previous behaviour) or threshold signing via the Signer trait
• create FrostSigner which has gRPC clients for the signing participants
• FrostSigner.sign() performs the signing process by calling each participant for their commitment (part 1) and signature share (part 2) and finally aggregates the shares to create a valid signature.
• FrostSigner.sign() will fail if it does not receive responses from at least the minimum number of signers required. the min number of signers is determined during key generation, which is done completely separately.

Testing

todo: testing with signer node (perhaps mocks as well?)

Changelogs

Ensure all relevant changelog files are updated as necessary. See
keepachangelog for change
categories. Replace this text with e.g. "Changelogs updated." or "No updates
required." to acknowledge changelogs have been considered.

Related Issues

closes #1937