rpc: add gettxproof and verifytxproof for transactions with witness

[Sjors]

The gettxproof and verifytxproof only proof Txid inclusion, not the witness. This PR adds that.

This PR introduces two new methods (gettxproof and verifytxproof), rather than expanding the originals, since those were designed around BIP37 bloom filters. The new methods require Wtxid instead of Txid. Clients that have the full serialized transaction can calculate both. Clients that only know the Txid have no need for the witness inclusion proof and can just keep using the old RPC methods.

Note that there is no -wtxindex (see #117) so we can't infer the block even when -txindex is enabled. If that's required, then see #118.

Commits:

• common: move merkle path helpers out of consensus: consensus/merkle.{h,cpp} already contains functions that are not actually used by consensus and are not needed for the kernel. Since PR introduces additional such functions, add common/merkle.{h,cpp}` for those.
• Allow calling GetWitnessCommitmentIndex with just the coinbase tx: taken from RPC/txoutproof: Support including (and verifying) proofs of wtxid bitcoin/bitcoin#32844, but renaming "generation" to "coinbase".
• common: add merkle branch verification helper
• common: add witness merkle path helper
• node: add transaction proof helpers: includes test fixtures of small and early blocks, which should probably be in a BIP too
• rpc: add verifytxproof
• `doc: add transaction proof RPC
• doc: release note

Superseedes bitcoin#32844

This PR may still contain LLM hallucinations, so I'm holding off on opening it on bitcoin/bitcoin. Feedback and testing is welcome, but detailed code review is probably premature.

TODO:

• move blockhash argument last for gettxproof to make (future) support of -wtindex easier (it becomes optional), see Witness transaction index #117

[Sjors]

Based on some light investigation, my current impression is that with Electrs you don't need -wtxindex (it also doesn't need -txindex), but with ElectrumX you would either need -wtxindex #117 or the wallet client needs to provide both txid and wtxid, so it can use the former to find the block.