Add support for exporting Signature objects as JSON

[asmeurer]

The format is compatible with the format produced by Griffe, except we don't use an ast form for the annotations.

[asmeurer]

Note that this relies on inspect.formatannotation for files that don't use from __future__ import annotations. Don't know how robust that is or if there could be weird annotation objects that it can't handle (although I would expect such things wouldn't be handled well by Sphinx either?)

[asmeurer]

Still need to output this to the actual data files, and add tests.

[Carreau]

Thanks, I had forgotten ammit had started this class. I'm guessing it is also not used everywhere and some place are still carrying str signatures.

[Carreau]

Thanks, merged and updated as part of #291, one change was that None is a valid annotation so I had to add a workaround between to not serialize and empty annotation as None. Otherwise both def f(a):pass and def f(a:None) would serialize as the same. This was present in annotation and default values. Of course in parameters annotation it's rare, but probably less in return annotations.