feat(io): Enable input from FASTA files

[roccomoretti]

To maximize the ease of use of RF3, it would be nice to be able to directly load in FASTA files (such that you can directly predict from a FASTA of your system). This PR puts together a potential system to add loading of systems from FASTA files, basically making it syntactic sugar around the JSON input format (though less powerful).

The file format is inspired by Boltz's FASTA input format, but slightly more flexible. Most fields are optional, and it should be robust to "extra" information in the label line. (You should be able to input most arbitrary polymeric FASTA files as-is and have them work, albeit without MSA ... which is also easy enough to add.)

While limited to FASTA input currently, it's written with an eye to be flexible for additional sequence file input formats, as desire dictates.

This is intended as a "draft" PR, for comment & feedback.