Replaces the (temporary) compile-time option to pass engine switches
with the ability to pass them temporarily at runtime via environment
variables. This moves the recently-added code for doing this on Windows
to a shared location for use by all desktop embeddings.
This is enabled only for debug/profile to avoid potential issues with
tampering with released applications, but if there is a need for that in
the future it could be added (potentially with a whitelist, as is
currently used for Dart VM flags).
Temporarily adds a way to enable mirrors as a compile time option,
as is already provided in the Linux embedding, to provide a migration
path for the one remaining known need for compile-time options
that has been raised in flutter/flutter#38569.
