Optional dependencies

See #15548 for an example of this problem.

Currently, we don't support optional dependencies in typeshed. Say a third-party package has some additional functionality if another third-party package is installed. For example, Authlib has a module authlib.integrations.requests_client that works only if requests is also installed. This leads to the awkward situation that our stubs for that module have many comments of the form "Inherits from requests.Session".

I suggest that we should add support for optional dependencies. Here's an idea:

• Add a field optional-requires to METADATA.toml. stub_uploader checks that this has the same limitations as requires.
• In tests optional-requires is treated the same way that requires is, i.e. the packages mentioned there are installed.
• stub_uploader adds this new field to project.optional-dependencies in pyproject.toml.

(Sidenote: Maybe we should rename requires to dependencies to match pyproject.toml.)

Linked PR:

• typeshed-internal/stub_uploader#203
• typeshed-internal/stub_uploader#208
• #15706

Start by reading the METADATA.toml handling and stub_uploader validation, then inspect the tests that install packages from requires. Define the optional-requires metadata behavior and verify that the relevant packages are installed for tests, while checking the referenced stub_uploader PRs and #15706 for existing decisions.