feat: support writing metadata

[henryiii]

Working toward #647. Starting by making it possible to go from Metadata back to RFC822.

[brettcannon]

I don't know if I'm comfortable with the JSON support. I actually almost had it in the first version of the module, but decided against it until we had a standard around it.

[henryiii]

Well, it is in PEP 566, so that's why I added it to pyproject-metadata. Though there's no use for it yet, so I can easily drop it.

I'll probably make a PR building on this PR for the next step.

This is starting as a fairly direct copy of pyproject-metadata, adapted for packaging's existing features and with the non-backward compatible changes listed in pypa/pyproject-metadata#140. I've got permission from the original copyright holders in pypa/pyproject-metadata#103 to contribute to packaging under the packaging license.

This PR also has the following changes vs. the original:

• All metadata keys are lower case, instead of title case. Headers in messages are not case sensitive.
• The ordering of the keys is based on the ordering in the Metadata class, which is slightly different compared to pyproject-metadata.

[henryiii]

FYI, on the flaking pypy-3.8 (and 3.9, possibly?) test, PyPy doesn't support pypy-3.8 or pypy-3.9 anymore. Only pypy-3.10 is actively supported.