multi: add BIP-322 support

[asheswook]

Change Description

Implements BIP-322.

Closes #2077

Details

• btcdutil/message: extract legacy message signing (BIP-137) into reusable package
• btcutil/bip322: BIP-322 implementation supporting P2WPKH, P2TR, P2SH-P2WPKH, P2PKH (sign + verify) and P2WSH (verify only)
• btcjson: add optional address parameter to SignMessageWithPrivKey
• rpcserver: SignMessageWithPrivKey supports BIP-322 signing when address is provided; verifymessage tries legacy first, then BIP-322

Steps to Test

cd btcutil/bip322 && go test ./... -v -count=1
cd btcjson && go test -run TestChainSvrCmds -v -count=1
go build ./...

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[asheswook]

CI fails with Failed to generate help for method 'signmessagewithprivkey': signmessagewithprivkey-address because of the TestHelp in rpcserverhelp.go. I'm gonna fix it.

[asheswook]

I fixed that and works well. Also more tests added

[ThomsenDrake]

Following the BIP-322 signing implementation in #2521 — this PR adds BIP-322 support to the RPC server layer. One question on the architecture: #2521 implements bip322 as a btcutil submodule with Sign/Verify at the library level, while this PR adds RPC server methods. Are these two efforts coordinated — i.e., does this PR use the btcutil/bip322 module from #2521, or implement signing independently?

From a btcpay-mcp integration standpoint, having BIP-322 signing available both as a library call (btcutil/bip322 Sign/Verify) and as an RPC (signmessagewithprivkey BIP-322 mode) would give us two useful abstractions. Library-level for agentic challenge-response auth (SEP-1686 flow), RPC-level for wallet-style message signing.

Curious whether the RPC server approach here and the library approach in #2521 are meant to complement each other or if this PR subsumes the library work.

[asheswook]

This PR is independent of #2521, adds the same implementation, but the two PRs are not related.
This is implements btcutil/bip322 module and adds RPC methods.

[asheswook]

It seems that other PR is going on merge, so I'm gonna close this.