Update the pants plugin development settings

Previously there was some missing pieces in the pants plugin development settings. For instance, `tools/pants-plugin.lock` did not include the necessary pantsbuild packages.

Following [the latest official guide](https://www.pantsbuild.org/2.21/docs/writing-plugins/overview) (for v2.21 we are using), I’ve updated the base configurations to make it working correctly.

### Main changes

- Update `pants.toml` to use separate Python interperter constraints for the `pants-plugins` resolve. Note that Pantsbuild uses Python 3.9 as its runtime.\* We no longer put `tools/pants-plugins` into the ignored dir/file list.

- Add `ruff.toml` to ensure Python 3.9 compatibility in the `tools/pants-plugins` directory when we run `pants lint check ::`. (e.g., Prevent it from converting nested f-string literals to use double-quotes to avoid syntax errors.)

JIRA Issue: BA-436