add gettxoutproof and verifytxoutproof commands and RPC calls

[markettes]

Description

Closes #2290

This change implements gettxoutproof and verifytxoutproof in btcd.

The change:

• adds merkle proof construction from explicit txids
• adds merkle proof extraction and validation for merkleblock proofs
• adds a blockchain helper for UTXO-based block lookup when blockhash is not provided
• wires both RPC commands into the RPC server and help output
• adds unit and RPC tests covering success and error paths

Implementation notes:

• gettxoutproof uses the explicit blockhash when provided
• otherwise it first attempts to resolve the block via the UTXO set
• if that does not succeed and txindex is enabled, it falls back to txindex
• verifytxoutproof validates proof structure and requires the referenced block
  to be in the active chain

Steps to Test

1. Run a btcd instance with --txindex and --simnet flags
2. Create a wallet with btcwallet and generate some blocks to fund it
3. Send some transactions and generate blocks to confirm them
4. Use gettxoutproof to retrieve a proof for a transaction, both with and without the blockhash argument
5. Use verifytxoutproof to verify the proof, ensuring it returns the correct transaction ID when valid and an error when invalid

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.

Code Style and Documentation

• The change is not insubstantial. Typo fixes are not accepted to fight bot spam.
• The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• Commits follow the Ideal Git Commit Structure.
• Any new logging statements use an appropriate subsystem and logging level.

[TechLateef]

The implementation is solid and I've verified the RPC behavior on simnet.
The UTXO → txindex fallback logic is well-structured. A few minor points i noticed:

Error Handling in handleVerifyTxOutProof

Currently, malformed proofs (e.g. decode failures, trailing bytes) return an
empty list []. It would be more idiomatic to return a
btcjson.ErrRPCDeserialization error for fundamentally invalid input, and
reserve the empty list for valid proofs that simply have no matches. This makes
it easier for callers to distinguish "bad proof" from "no match".

Lookup Inconsistency in resolveTxOutProofBlock

The UTXO lookup path is best-effort — it skips missing transactions and
continues. The txindex fallback however returns an error immediately if a
single transaction is missing. For batch queries this inconsistency can be
confusing. Aligning both paths to the same strategy (either fail-fast or
best-effort) would improve predictability.

Missing Bounds Check in ExtractMatches

It would be safer to validate that msg.Transactions does not exceed
maxTxPerBlock, consistent with how other block-related logic handles this.
This guards against malformed or malicious input reaching deeper into the
proof verification logic.

Missing Docstrings

A few of the new unexported helper functions added in rpcserver.go are
missing doc comments. Adding them keeps the codebase consistent and helps
future contributors understand intent without having to trace the logic.

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Great work overall on this!