miner: add package feerate vector to CBlockTemplate

- The package feerates are ordered by the sequence in which
  packages are selected for inclusion in the block template.

- The commit also tests this new behaviour.

Co-authored-by: willcl-ark <will@256k1.dev>

src/node/miner.cpp

@@ -421,6 +421,7 @@ void BlockAssembler::addPackageTxs(int& nPackagesSelected, int& nDescendantsUpda
         }
 
         ++nPackagesSelected;
+        pblocktemplate->m_package_feerates.emplace_back(packageFees, static_cast<int32_t>(packageSize));
 
         // Update transactions that depend on each of these
         nDescendantsUpdated += UpdatePackagesForAdded(mempool, ancestors, mapModifiedTx);

src/node/miner.h

@@ -10,6 +10,7 @@
 #include <policy/policy.h>
 #include <primitives/block.h>
 #include <txmempool.h>
+#include <util/feefrac.h>
 
 #include <memory>
 #include <optional>
@@ -39,6 +40,9 @@ struct CBlockTemplate
     std::vector<CAmount> vTxFees;
     std::vector<int64_t> vTxSigOpsCost;
     std::vector<unsigned char> vchCoinbaseCommitment;
+    /* A vector of package fee rates, ordered by the sequence in which
+     * packages are selected for inclusion in the block template.*/
+    std::vector<FeeFrac> m_package_feerates;
 };
 
 // Container for tracking updates to ancestor feerate as we include (parent)