[MNT] Identify modified extra(s) in PR's that modify pyproject.toml

[yarnabrina]

As part of #5375, a new CI workflow is introduced to validate extras. This works fine, but triggers a lot of jobs (135 = 9 extra × 5 python version × 3 operating systems) for ANY change in pyproject.toml.

It'd be nice to validate only modified extras instead of checking all, which is already being done as part of a scheduled workflow.

Desired behaviour:

1. No job as part of this workflow if no component specific extras are modified.
2. If one or more component specific extra gets modified, only those are detected. Then the validation works only for those and ignores rest.

Suggestions:

1. Add a new job to read pyproject.toml file and detect the changes. This will output a JSON or similar output with true or false corresponding to each extra.
2. The new job loops over this output and only modified ones get triggered.

This suggestion is along the lines of "Use change detection to configure matrix job" example from here. This is not necessarily the only way, and any improvements or completely different approaches are very much welcome.