Provide a default /health endpoint

Current situation

Some services have endpoints that allow for a ?ping query parameter, which can serve as a healthcheck, but this is an adhoc solution.

Desired outcome

All service should have a /health endpoint, returning a 200 status code (with an OK value).

It should be possible to open this endpoint to unauthorized requests (disabled by default).

A enable_insecure_healthcheck_endpoint context parameter is available (yes, true, on, 1). A command-line parameter can override it.

An environment variable OPENTF_ENABLE_INSECURE_HEALTHCHECK_ENDPOINT (same values, case insensitive) can be use. It overrides the context parameter value but does not override the command-line parameter (command line parameter > env variable > context parameter).