Merge bitcoin/bitcoin#30955: Mining interface: getCoinbaseMerklePath() and submitSolution()

525e9dcba0 Add submitSolution to BlockTemplate interface (Sjors Provoost)
47b4875ef0 Add getCoinbaseMerklePath() to Mining interface (Sjors Provoost)
63d6ad7c89 Move BlockMerkleBranch back to merkle.{h,cpp} (Sjors Provoost)

Pull request description:

  The new `BlockTemplate` interface introduced in #30440 allows for a more efficient way for a miner to submit the block solution. Instead of having the send the full block, it only needs to provide the nonce, timestamp, version fields and coinbase transaction.

  This PR introduces `submitSolution()` for that. It's currently unused.

  #29432 and https://github.com/Sjors/bitcoin/pull/48 use it to process the Stratum v2 message [SubmitSolution](https://github.com/stratum-mining/sv2-spec/blob/main/07-Template-Distribution-Protocol.md#77-submitsolution-client---server). The method should be sufficiently generic to work with alternative mining protocols (none exist that I'm aware off).

  This PR also introduces `getCoinbaseMerklePath()`, which is needed in Stratum v2 to construct the `merkle_path` field of the `NewTemplate` message (see [spec](https://github.com/stratum-mining/sv2-spec/blob/main/07-Template-Distribution-Protocol.md#72-newtemplate-server---client)). The coinbase merkle path is also used in Stratum "v1", see e.g. https://bitcoin.stackexchange.com/questions/109820/questions-on-merkle-root-hashing-for-stratum-pools

  This last function uses `BlockMerkleBranch` which was moved to the test code in #13191. The reason back then for moving it was that it was no longer used. This PR moves it back.

  This PR does not change behaviour since both methods are unused.

ACKs for top commit:
  achow101:
    ACK 525e9dcba0
  itornaza:
    Code review ACK 525e9dcba0
  tdb3:
    Code review and light test ACK 525e9dcba0
  ryanofsky:
    Code review ACK 525e9dcba0. Left minor suggestions but none are important, and looks like this could be merged as-is

Tree-SHA512: 2a6a8f5d409ff4926643193cb67702240c7c687615414371e53383d2c13c485807f65e21e8ed98515b5456eca3d9fca13cec04675814a4081467d88b849c5653

src/interfaces/mining.h

@@ -42,6 +42,20 @@ public:
     virtual CTransactionRef getCoinbaseTx() = 0;
     virtual std::vector<unsigned char> getCoinbaseCommitment() = 0;
     virtual int getWitnessCommitmentIndex() = 0;
+
+    /**
+     * Compute merkle path to the coinbase transaction
+     *
+     * @return merkle path ordered from the deepest
+     */
+    virtual std::vector<uint256> getCoinbaseMerklePath() = 0;
+
+    /**
+     * Construct and broadcast the block.
+     *
+     * @returns if the block was processed, independent of block validity
+     */
+    virtual bool submitSolution(uint32_t version, uint32_t timestamp, uint32_t nonce, CMutableTransaction coinbase) = 0;
 };
 
 //! Interface giving clients (RPC, Stratum v2 Template Provider in the future)