Closes #10199.
Towards #10182.

Preview only for now to show the direction this is going. There are still quite a few potentially blocking code paths that are not covered yet.

Notes

• ACL._ensure_loaded() has a default timeout, but not all internal calls to it can be made that configurable, such as in ACL.__iter__(). I thus left that part to use a default timeout.
• Blob.download_to_file(), Blob.upload_from_file(), and the methods that rely on these two do not expose timeouts, because the underlying resumable-media logic applies its own default timeout to transport.request(), which would override user timeouts.
  Getting around that would first require adding configurable timeouts to the resumable-media dependency (there is a PR #116 that adds these, but only for uploads and it's not merged yet) .

Things still to do/check:

• Add timeout to the remaining methods that use client._connection.api_request() (read: helpers)
• Add timeout to any methods that use _connection._make_request() directly, i.e. bypassing the api_request() method.
• Add timeout to public methods that interact with methods using the timeout (to pass the timeout through).

PR checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)