feat: add variant schema

[swarnimarun]

TODO: need to add inline documentation.

[pavelzw]

Closes prefix-dev/rattler-build#295

All compilers and compiler versions are still missing. Also we need macos deployment target. Is that already incorporated into rattler-build?

[wolfv]

@pavelzw I don't think compilers are in any way "mandatory" - they are just another variant config value. How do you envision them to be added?

[pavelzw]

Maybe something like this?

compilers:
  cpp:
    name/package/???: clang
    version: '12.0'
  <any-key>:
    ...

Otherwise just an entry for the most common compilers c, cpp, rust, go, go-nocgo, ... to make auto completion easier

[wolfv]

@pavelzw I like it – but this might need a CEP etc.

I also think we could add another nesting level so that we have:

zip_keys:
 ...
pins:
 ...
compilers:
 ...
pin_run_as_build:
 ...

But again, we haven't actually done any thinking on it yet.

Currently the compiler are as they used to be

compiler_cxx: ...
compiler_c: ...

[pavelzw]

Not sure how you guys feel about this, how about changing this to some link at prefix.dev that is a 300 to this URL? This would allow you to use better versioning and not be bound by this specific location in the repo if you ever decide on moving/renaming these files.