signrpc: update to musig2 0.4.0

[Roasbeef]

Right now our implementation of musig is fully compliant up until version 0.2.0. Since then, a few changes have been made to nonce generation (which doesn't directly impact the signature generation, and can't fully be verified unless musig-DN is used):

0.5.0 (2022-06-21): Separate ApplyTweak from KeyAgg and introduce KeyGen Context.
0.4.0 (2022-06-20): Allow the output of NonceAgg to be infinity and add test vectors
0.3.2 (2022-06-02): Add a lot of test vectors and improve handling of invalid contributions in reference code.
0.3.1 (2022-05-24): Add NonceGen test vectors
0.3.0 (2022-05-24): Hash i - 1 instead of i in NonceGen
0.2.0 (2022-05-19): Change order of arguments in NonceGen hash function
0.1.0 (2022-05-19): Publication of draft BIP on the bitcoin-dev mailing list  

We should implement the changes to nonce generation, and catch up our set of test vectors.

A PR that catches btcec up to 0.4.0 can be found here: btcsuite/btcd#1865

[Roasbeef]

Ok the dependent PR has been merged.

@sputn1ck now we need a PR to lnd that upgrades to the following btcd modules:

• btcutil/psbt/v1.1.5
• btcutil/v1.1.2
• btcec/v2.2.1