rpc: add coinbase_tx field to getblock

This adds a "coinbase_tx" field to the getblock RPC result, starting
at verbosity level 1. It contains only fields guaranteed to be small,
i.e. not the outputs.

doc/release-notes-34512.md

@@ -0,0 +1,8 @@
+Updated RPCs
+------------
+
+- The `getblock` RPC now returns a `coinbase_tx` object at verbosity levels 1, 2,
+  and 3. It contains `version`, `locktime`, `sequence`, `coinbase` and
+  `witness`. This allows for efficiently querying coinbase
+  transaction properties without fetching the full transaction data at
+  verbosity 2+. (#34512)