List optional dependencies in pyproject (apart from docs and tests)

[radarhere]

In thinking about #7502, it occurred to me that listing olefile and defusedxml as optional dependencies in pyproject.toml is probably helpful, as a form of documentation.

[hugovk]

Sounds reasonable, people will be able to install like this:

python -m pip install pillow[fpx_mic]  # also installs olefile
python -m pip install pillow[xmp]      # also installs defusedxml

Shall we replace fpx_mic with fpx and mic?

python -m pip install pillow[fpx]  # also installs olefile
python -m pip install pillow[mic]  # also installs olefile
python -m pip install pillow[xmp]  # also installs defusedxml

Let's bikeshed now, because if we change it later, some may see it as a breaking change :)