Extend `RunReactNativeXamlCodeGen` target to support multiple WinMDs

[jonthysell]

Summary

As of PR #234 , there is a target named RunReactNativeXamlCodeGen which runs the codegen as part of the RNX native build.

Right now it only supports specifying one WinMD file, but the Codegen can actually support more, so we should allow users to specify more.

Motivation

If a user wants to make sure that Codegen is run, it's probably because they want to add multiple WinMDs from custom control libraries they want to add.

Basic Example

No response

Open Questions

No response

[chrisglein]

Current customers should be unblocked with the linked PR. Will wait to prioritize supporting multiple for when there's more ask for that use case.