Explore creating `packages/testing-types`

There's a potential benefit of extracting types used in both `packages/testing` and `src/ethereum_spec_tools` and condense them in a dedicated package that can be imported by both, primarily to try to minimize the JSON serialization round-trip performed in each state transition request from the tests to the T8N.

Particularly types contained in `packages/testing/src/execution_testing/client_clis/cli_types.py` have the most potential to be reused.

Creating this tracking issue to discuss potential downsides to this and any walls that could be hit during the process.