mark pydantic integration (as an extra?)

We run mypy with ignore_missing_imports = false and then have an explicit allow list for packages. Since cloudpathlib imports pydantic (albeit behind a guard) but does not depends on it, mypy fails with :

cloudpath.py:1207: error: Cannot find implementation or library stub for module named "pydantic_core"  [import-not-found]

We would normally set ignore_missing_imports for just pydantic_core, but that's somewhat wonky in this case because pydantic_core does have type stubs, it's just not a dependency.

I think the most straightforward approach would be to add a pydantic extra to cloudpathlib, but there might be a fancier way to inform mypy directly of the "optional-ness" of the imports.