Allow to customize FUZZ_SECONDS option per target

First thank you so much for clusterfuzzlite. I have a great time using it in [Oxigraph](https://github.com/oxigraph/oxigraph).

Oxigraph provides multiple targets, some fuzzing simple parsers and one fuzzing the query optimizer. The query optimizer target is way slower than the parsing ones, requirering much more time to yield interesting results or even to go through the already existing corpus. I would love a way to set different time limit per target or have an option in the `google/clusterfuzzlite/actions/run_fuzzers` GitHub to only run a specific list of targets instead of all of them.