Changes necessary for Stateless Witness Building

[jwasinger]

This implements:

• The necessary changes to construct a stateless block witness.
• A debug API method debug_buildStatelessProof to construct a witness for a given block.
• A blockchain test runner which constructs a witness for each successfully-imported block.

This PR pulls changes from #28776, omitting:

• stateless execution implementation, and the standalone utility/sub-command which exposes a stateless block execution endpoint behind an HTTP server.
• stateless cross-validation logic in core/blockchain.go during block import.

[jwasinger]

I still need to fix the unit tests broken by this.