SOS/dotnet-dump extensibility

Criteo has been doing some great stuff with CLRMD over the years and it would be nice if they and others could use their analysis tools with dotnet dump. Initial scope would just be the ability for a 3rd party to grab a library from NuGet, code an extension against it, and then have a way to load that extension into the dotnet-dump analyze REPL.

We have one prototype I made of what that might look like [here](https://github.com/noahfalk/diagnostics/tree/example_analyzer_extensibility) but I don't think anyone should read to much into it until @mikem8361 has had an opportunity dig into it.

We've got a bunch of runtime work that will be important to get done prior to missing our 3.0 ship window whereas this work can be done out-of-band at any time. I imagine this will mean the extensibility work doesn't get prioritized for a little while yet, but I wanted to get this issue in place to track it.

cc @chrisnas @mikem8361