Simplify SOS' discovery/hosting of CoreCLR by using nethost + hostfxr

## Background and Motivation

Today, SOS manually directly calls the CoreCLR hosting APIs on coreclr.dll/libcoreclr.so. This requires is to do a significant amount of work to locate CoreCLR and construct valid hosting parameters. This also means it does not benefit from any improvements in the hosting layer (of which there have been a few in recent years).

## Proposed Feature

Rewrite hostcoreclr.cpp to use `nethost` (or `libnethost`) to discover and load the .NET runtime for the managed components of SOS.

## Usage Examples