Support strongly typed entity invocation

[jviau]

This issue tracks the discussion and design for adding a strongly typed pattern for entity invocation. For in-proc durable functions, entities can be strongly invoked via dynamic proxies. This is an option for dotnet isolated, however, we should implement it without using 3rd party libraries (keep our dependencies minimal). An alternative would be to perform this via a source generator. But we should not limit the discussion to only proxies and also evaluate alternatives (such as mediator pattern #73).

TODO: flush out options, requirements