rpc: Add getblockfilter RPC method.

Retrieves and returns block filter and header from index.

test/functional/test_runner.py

@@ -144,6 +144,7 @@ BASE_SCRIPTS = [
     'wallet_txn_doublespend.py',
     'wallet_txn_clone.py --mineblock',
     'feature_notifications.py',
+    'rpc_getblockfilter.py',
     'rpc_invalidateblock.py',
     'feature_rbf.py',
     'mempool_packages.py',