[ENH]: Add types for fig_kw argument in subplots #31003

Problem

In pandas-stubs, we are looking to support pyright strict typing checks.

Because the argument fig_kw is untyped in the declaration of subplots in pyplot.py, pyright will report reportUnknownMemberType with strict type checking.

See https://github.com/pandas-dev/pandas-stubs/pull/1639/changes/de0a4176d4704d81cebd44cd201382340d0c2fbb..7985c9b0b8ec0dedca3fae36a1e539a386bdee27#r2709077432 for relevant comment

Proposed solution

Annotate **fig_kw as Any in the various subplots declarations.