feat!: only use uptime_check_auth_proxy when needed

This is really a bugfix but creates a breaking change, thus
the need for a new major version release.

When trying to enable this module with authentication_proxy = { enabled = false }
the module module.uptime_check_auth_proxy is executed
despite not being needed.

Add a conditional to this module so that it is only executed
when needed. This will cause deletion of elements in projects
using the 3.x version of this module, thus the introduction of
a new major version.

Also tidy up the various locals, outputs and resources which depend on
module.uptime_check_auth_proxy to understand that the proxy may not be
present depending on configuration.

More details in #13