add GetExpressionTree in ActivatorFactory

[joelverhagen]

Interesting, does this demonstrate better perf than some of the other approach? Last time I checked IL emit was the fastest but had an extreme first time cost. Where does this approach fall in the startup perf vs. per-invocation perf?

Happy to merge it either way to demonstrate a variety of approaches.

[leandromoh]

Expression tree looks to be the fastest... at least on .net 5. See https://andrewlock.net/benchmarking-4-reflection-methods-for-calling-a-constructor-in-dotnet/