COM Activation host (dotnet/core-setup#4577)

COM Activation host (dotnet/core-setup#4577)

* Add new comhost library for COM activation
 - Supports reading clsidmap as file or resource
 - Supports registration and unregistration of COM server

* Replace COREHOST_MAKE_DLL define with EXPORT_SHARED_API

* Convert CoreClr instance loading API from a flat C-style to a class API . This will make having multiple CoreClr instances in the same process easier in the future.

* Move internal execute* APIs to the fx_muxer.cpp compilation unit
Add initial/erroring implementation for get_com_activation_delegate() to fx_muxer_t.

* Create coreclr_property_bag_t for property management.

* Extract coreclr property creations into a reusable class.

* Report process ID during test run.

* Remove the new corehost_ensure_load() export since the previous semantics
  of reinitializing hostpolicy were designed for testing.

* Fix P/Invoke signature for hostfxr_get_native_search_directories to define
  the calling convention and remove usage of StringBuilder.

* Properly propagate the host command on reinitialization
Remove additional uses of global variables where possible

* Don't add the COM server assembly to the TPA
Ensure the COM server assembly deps.json isn't added to the AppContext

* Feedback on error propagation

Commit migrated from https://github.com/dotnet/core-setup/commit/1d95ece515edbbad0ef8f8980424d3e3bce49cb2