feat(fill): Allow to define which formats to fill (`--formats=<str>` flag)

This PR achieves our immediate goals for `benchmark` and full releases. But I think the interface should be made more flexible/generic. Can also be done in a follow-up PR.

Ideas:
- Define `DEFAULT_FIXTURE_FORMATS` for the formats that get generated without any flag (as of now that's state, blockchain and blockchain engine fixtures).
- Add a `--formats=` flag to specify which formats to generate (a comma-separated list of format names?) We could potentially use our pytest marks for this purpose, but this is clumsy, as they need to be combined via `or`, in particular in combination with other marks, e.g. `slow` or `benchmark`.

_Originally posted by @danceratopz in https://github.com/ethereum/execution-spec-tests/pull/1855#discussion_r2189143853_