Adds a new ServiceDefaults library (currently entirely OTel-related).

This is extremely WIPish, it's just easier to get the package into the container if I can publish to nuget as I iterate. I know I need to solve this problem better longterm, it's just easier to get the package source into the container env if it's on NuGet for now.

To address a couple of likely questions:

• Not worrying about default health checks or service discovery right now, but we can return to this as they become useful. The short-term goal is to package up easy writing to OTel.
• (Optionally) Using Prometheus: - I was very hesitant to do this originally, but optionally using the Prometheus exporter is one of the easier ways to more generally publish a /metrics endpoint that scrapers like Prometheus can consume. I thought it might be useful in debugging as we get this up and rolling, to make sure we're writing the metrics we want to. I see this being off most of the time in practice except for specialized cases, or to check stuff out in dev.

I expect this will change/grow a lot even over the course of my initial implementation, but wanted to get something in there. Very much welcome your thoughts.