Allow customization of the StartupHook dll

[pjanotti]

trafficstars

This is more to facilitate development and experimentation and perhaps an extension point for vendors. See https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation/pull/314#discussion_r798054857 for the context.

The profiler code currently assumes that the StartupHook has a particular name, and is located in a specific installation directory. We might want to break some of these assumptions to support different experiments and vendor extension points. Perhaps there's an additional experimental startup hook, or perhaps a vendor wants to use their own startup hook.

[pellared]

SIG meeting

@nrcventura to describe it better post-beta