Load agent with startuphook environment variable

Follow up from https://github.com/elastic/opbeans-dotnet/pull/34#discussion_r557278794

Currently updating the .NET Agent version needs changes at multiple places. Idea: let's see if it makes sense to load the agent with [zero code change](https://www.elastic.co/guide/en/apm/agent/dotnet/current/setup-dotnet-net-core.html#zero-code-change-setup) injection for opbeans-dotnet. With that we'd get rid of the need to update direct references.