Provide alternative choices for JSON libraries

[max-moser]

Once again, I've been doing some experimentation with free-threading Python in the hopes of simplifying our setup by improving the situation regarding the limited number of Python workers on the web server.

As part of this work, I stumbled upon the blockers that prevented the use of free-threading Python:

• The binary distributions of psycopg don't support free-threading Python, but the source build does
• orjson does not support free-threading Python either, and there doesn't seem to be a clear timeline; the issue tracker and PR tabs on the GitHub project have also been closed now (with the README stating that was due to the signal-to-noise ratio)...

In this (still very rough and unpolished) PR, I've implemented support for a few alternative libraries to handle JSON, via extras (e.g. commonmeta-py[msgspec]:

• orjson: kept support for that via commonmeta-py[orjson]
• ujson: similar to orjson; has free-threading support, but is a bit slower and in maintenance mode (even the maintainers suggest moving to orjson if possible)
• msgspec: a performant alternative [1], with slightly different API
• [json]: if no other implementation was chosen, fall back to the standard library

[1] https://msgspec.dev/benchmarks#json-serialization

---

Note that msgspec does not like Python 3.9:

  x No solution found when resolving dependencies for split (markers: python_full_version == '3.9.*'):
  `-> Because the requested Python version (>=3.9) does not satisfy Python>=3.10 and msgspec>=0.21.0 depends on Python>=3.10, we can conclude that msgspec>=0.21.0 cannot be used.
      And because only the following versions of msgspec are available:
          msgspec<=0.21.0
          msgspec==0.21.1
      we can conclude that msgspec>=0.21.0 cannot be used.
      And because commonmeta-py[msgspec] depends on msgspec>=0.21.0 and your project requires commonmeta-py[msgspec], we can conclude that your project's requirements are unsatisfiable.

hint: While the active Python version is 3.14, the resolution failed for other Python versions supported by your project. Consider limiting your project's supported Python versions using `requires-python`.