Allow users to specify which distro and configurator to use in auto instrumentation

[jeremydvoss]

Is your feature request related to a problem?
Yes. Currently, if a user has multiple packages with distro or configurator entry points installed, they have no control over which are used for the auto instrumentation flow.

Describe the solution you'd like
Copying what we do for exporters, I am adding 2 env vars where users can specify the entry point names of the distros and configurators they want used. I added warnings for each distro and configurator not used.

Describe alternatives you've considered
I considered requiring thes environment variables to be used but instead chose to make them optional. If the env vars are not set, then existing behavior continues: the first distro and configurators found will be used.

Additional context
With the way the package is set up, this also adds the --distro and --configurator command line args. Also, we currently have not tests for the sitecustomize file. I found it tricky to implement because importing the module triggers instrumentation. Open to suggestions.