[Rust/Bitcoin/Utxo] Create tw_utxo and Proto structures

[lamafab]

First part of #3339 (still have a bunch of comments to add)

My working branch is messy right now, so this branch only contains the changes related to tw_utxo, plus the Utxo.proto. Will keep merging changes from my working branch to this branch. Regarding the tests: I just reused the ones in tw_bitcoin, but ideally I should create clean transactions using bitcoind -regtest.

[MaximPestryakov]

Noob question. Shouldn't we also have P2SH here?

[lamafab]

This method (which I now renamed to signing_method) is only from the perspective of the Utxo structure which is not aware of any transaction standards (such as P2PKH, P2WPKH, etc). That field only tells this crate how to sign the input, such as "legacy" for P2SH/P2PKH, "segwit" for P2WPKH, etc. The Bitcoin input structure will look like this:

message Input {
	bytes txid = 1;

	uint32 vout = 2;

	uint64 amount = 3;

	Utxo.Proto.SighashType sighash_type = 4;

	oneof to_recipient {
		Builder builder = 5;
		ScriptWitness custom = 6;
	}

	// TODO: This should only be exposed for taproot
	bool one_prevout = 7;

	message Builder {
		oneof variant {
			// Pay-to-Script-Hash, specify the redeem script.
			bytes p2sh = 1;
			// Pay-to-Public-Key-Hash, specify the public key.
			bytes p2pkh = 2;
			// Pay-to-Witness-Script-Hash, specify the redeem script.
			bytes p2wsh = 3;
			// Pay-to-Public-Key-Hash, specify the public key.
			bytes p2wpkh = 6;
			// Pay-to-Taproot-key-path (balance transfers), specify the public key.
			bytes p2tr_key_path = 7;
			// Pay-to-Taproot-script-path (complex transfers)
			TaprootScriptPath p2tr_script_path = 8;
			Brc20Inscription brc20_inscribe = 9;
		}
	}

	message ScriptWitness {
		bytes script_sig = 1;
		repeated bytes witness_items = 2;
	}

	message TaprootScriptPath {
		bytes payload = 1;
		bytes control_block = 2;
	}

	message Brc20Inscription {
		bytes inscribe_to = 1;
		string ticker = 2;
		uint64 transfer_amount = 3;
	}
}

I will update everything of Friday evening.

[lamafab]

Closing in favor of #3382