Add `fill` flag to explicitly specify the output fixture formats

This means it'll be impossible to generate `blockchain_test_engine` fixtures from a benchmark test. Which is fine as no one uses it! 👍

Tbh, in general though, I would love to have an explicit and clean way to tell `fill` which outputs to generate. Currently, the only real way to specify fixture format outputs is via `-m` which is a bit clumsy, so I'd prefer something like `--formats=blockchain_test,blockchain_test_enginex`. For another PR.

I would prefer this kind of change to be a smaller, targeted PR, it gets a bit lost here.

_Originally posted by @danceratopz in https://github.com/ethereum/execution-specs/pull/2888#discussion_r3302064512_