add option to disable smart agent bundle use

[atoulme]

Fixes #4

This pull request allows a user to explicitly disable the Smart agent bundle download, to save on disk space.

1. Remove the existing buildpack if you have installed it in the past:
   heroku buildpacks:remove https://github.com/signalfx/splunk-otel-collector-heroku.git\#v0.0.1
2. Add a new configuration entry to explicitly disable the smart agent download:
   heroku config:set SFX_AGENT_DISABLED=true
3. Add the new buildpack:
   heroku buildpacks:add https://github.com/atoulme/splunk-otel-collector-heroku.git\#disable_sfx_agent
4. Push a release like usual. Check the output of the push, looking for this line:
   remote: -----> Skipping installing the SignalFx Agent Bundle

That's it! You should be set.
Note disabling the smart agent has side effects. We will no longer collect heroku metadata about your application. I have opened open-telemetry/opentelemetry-collector-contrib#16833 to create a native receiver to replace this monitor going forward.