Allow passing hardcoded uid to AuthExternal

[kiwiz]

Is your feature request related to a problem? Please describe.

I'm trying to connect to the system DBus within a Docker container that is utilizing a user namespace. This fails, as the os.uid() call here returns the remapped UID (which is entirely expected).

Describe the solution you'd like

For my use case, I would like to pass a hardcoded UID to AuthExternal, perhaps via the constructor or an env variable. If this sounds like a reasonable change, I can submit a PR.

Additional context

I arrived here via the following path:

• home-assistant:bluetooth
• bleak
• dbus-fast

Let me know if this feature request would be better suited for one of those repos.

[bdraco]

We can accept a PR for this via the constructor as an optional keyword arg as long as it has test coverage 👍