Change instrumentation feature gates into normal command-line flags: operator.autoinstrumentation.dotnet

[jaronoff97]

We currently use operator feature gates to control which instrumentations are enabled. The intent is to keep this configurable forever, so cluster operators can make this decision for the whole operator deployment. Feature gates are a suboptimal choice for this, however, as they assume a strict stage progression from Alpha to Stable, and eventually are intended to be removed completely.

We should change these into normal command-line flags for the operator while it's still a relatively painless change for our users.

see #2582