Healthcheck endpoint for workers

**Description**

Now that the worker only responds to requests signed by the webserver (#16754), we should consider adding a health endpoint to the worker to help ensure it is up and healthy.

**Use case / motivation**

Users may want to have external monitoring on the workers logging webserver and having something that responds with a 200 instead of a 403 would be handy.

https://github.com/apache/airflow/pull/16754#issuecomment-885661439

**Related Issues**

#16574

Start by reading the worker request-handling changes in #16754 and the related issue #16574 to understand the current signed-request behavior. Define and implement the health endpoint so external monitoring receives HTTP 200 while ordinary worker requests remain protected, then verify both responses with the relevant worker tests.

Written by the indexing model from the issue text.