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);