Secp256k1 - Tweak implementation

[mmtftr]

Proposal Description

Todos:

• Implement tweaking of Agg Public Key from Merkle root (s) -- generate scalar and call secp pubkey_xonly_tweak
• Figure out: how does the secp tweak function work? (Is it as we expect, which is Tweak(IPK, t) = IPK*
• Figure out: how to create a signature valid for a tweaked musig2 public key?

[mmtftr]

Closing this issue as the implementation with tests were added in two commits to #405. The test demonstrates that the new key_agg_cache correctly creates a signature for the tweaked aggregate public key.

We haven't figured out the secp tweak internals but this BIP describes how tweaked multisig is supposed to work for reference.

Please refer to #339 for parent work.