Allow the user to use custom signing scheme and multiple keys in vtxo tree signatures

[louisinger]

This PR adds a way for the user to sign only its branch during the vtxo tree signing process.

now RegisterOutputsForNextRound RPC expects the following request:

message Musig2 {
  repeated string cosigners_public_keys = 1;
  uint32 signing_type = 2;
}

message RegisterOutputsForNextRoundRequest {
  string request_id = 1;
  // List of receivers for to convert to leaves in the next VTXO tree. 
  repeated Output outputs = 2;
  optional Musig2 musig2 = 3;
}

• ephemeral_pubkey has been replaced by musig2.cosigners_public_keys. It allows setting multiple signing keys for the same request. Useful for collaborative VTXO contracts.
• signing_type is a flag signalling to the server the type of signing scheme the user wants to use, 2 are supported:
  • SignAll = 0 is the existing scheme where one signs all branches of the tree.
  • SignBranch = 1 is the new scheme where one signs only the branch(es) related to his VTXO(s) in the tree.

BREAKING CHANGES:

With this, the tree's PSBTs don't embed the sweep tapscript leaf and internal taproot key as unknown fields anymore. They are replaced instead with the batch lifetime, as clients know all other info to reconstruct the sweep leaf on their own.

@tiero @altafan @sekulicd please review

[altafan]

Tried to make 2 rest client settle at the same time and they crashed because of the encoding of the nonces. Seems like some elems of the matrix are nil when signing a branch.