interfaces: refactor: move submitSolution implementation to miner

- Create a new function `AddMerkleRootAndCoinbase` that compute the
  block's merkle root, insert the coinbase transaction and the merkle
  root into the block.

src/node/interfaces.cpp

@@ -929,22 +929,8 @@ public:
 
     bool submitSolution(uint32_t version, uint32_t timestamp, uint32_t nonce, CTransactionRef coinbase) override
     {
-        CBlock block{m_block_template->block};
-
-        if (block.vtx.size() == 0) {
-            block.vtx.push_back(coinbase);
-        } else {
-            block.vtx[0] = coinbase;
-        }
-
-        block.nVersion = version;
-        block.nTime = timestamp;
-        block.nNonce = nonce;
-
-        block.hashMerkleRoot = BlockMerkleRoot(block);
-
-        auto block_ptr = std::make_shared<const CBlock>(block);
-        return chainman().ProcessNewBlock(block_ptr, /*force_processing=*/true, /*min_pow_checked=*/true, /*new_block=*/nullptr);
+        AddMerkleRootAndCoinbase(m_block_template->block, std::move(coinbase), version, timestamp, nonce);
+        return chainman().ProcessNewBlock(std::make_shared<const CBlock>(m_block_template->block), /*force_processing=*/true, /*min_pow_checked=*/true, /*new_block=*/nullptr);
     }
 
     std::unique_ptr<BlockTemplate> waitNext(BlockWaitOptions options) override

src/node/miner.cpp

@@ -434,4 +434,17 @@ void BlockAssembler::addPackageTxs(int& nPackagesSelected, int& nDescendantsUpda
         nDescendantsUpdated += UpdatePackagesForAdded(mempool, ancestors, mapModifiedTx);
     }
 }
+
+void AddMerkleRootAndCoinbase(CBlock& block, CTransactionRef coinbase, uint32_t version, uint32_t timestamp, uint32_t nonce)
+{
+    if (block.vtx.size() == 0) {
+        block.vtx.emplace_back(coinbase);
+    } else {
+        block.vtx[0] = coinbase;
+    }
+    block.nVersion = version;
+    block.nTime = timestamp;
+    block.nNonce = nonce;
+    block.hashMerkleRoot = BlockMerkleRoot(block);
+}
 } // namespace node

src/node/miner.h

@@ -229,6 +229,9 @@ void RegenerateCommitments(CBlock& block, ChainstateManager& chainman);
 
 /** Apply -blockmintxfee and -blockmaxweight options from ArgsManager to BlockAssembler options. */
 void ApplyArgsManOptions(const ArgsManager& gArgs, BlockAssembler::Options& options);
+
+/* Compute the block's merkle root, insert or replace the coinbase transaction and the merkle root into the block */
+void AddMerkleRootAndCoinbase(CBlock& block, CTransactionRef coinbase, uint32_t version, uint32_t timestamp, uint32_t nonce);
 } // namespace node
 
 #endif // BITCOIN_NODE_MINER_H