Allow load()ing .toml files from starlark

[zsol]

This PR allows writing:

load(":foo.toml", "value")
print(value)

The toml file is mapped to JSON, and then exposed the same way static JSON files are to starlark code. The single value name in a toml file will always be a dictionary (as TOML files are always tables).

TODO:

• write e2e tests that are exercised in OSS CI
• write docs

[facebook-github-bot]

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[thoughtpolice]

I've wanted this kind of feature literally a dozen times now, I think, and was recently thinking of (re?)proposing it; thank you so much for it!

[zsol]

@JakobDegen wdyt?

[JakobDegen]

Didn't see this

@zsol can you write up an API change proposal that I can copy-paste into Buck2 Dev? Doesn't need to be long, just enough to mention that this is a close analogue to the json case and then if you made any potentially controversial decisions mention those.