Add oslo.config.opts entrypoint for basic auth middleware

... so that its options can be rendered by oslo-config-generator by
adding the entry point.
This change also updates the base oslo.middleware entry point to
include options of the middleware because the base entry point is
supposed to include all options in the oslo.middleware library.

Closes-Bug: #1940747
Change-Id: Ic9d79d9c46fc1dc78aa4d089e36219b2a34f4099