FetchContent: Support setting cache variables as part of the declared details
This has been split out from the discussion thread at #21687 (comment 936429).
In essence, the suggested feature is to allow
FetchContent_Declare() to specify
key=value variables that would be set for the scope in which
add_subdirectory() is called. This would be used to influence the adding of just that dependency.
It should be pointed out that such variables would be visible to any child dependencies that the immediate dependency pulls in. It would also probably be necessary to implement the new policy/behavior proposed in #22038 (comment 934910) for this to be doable safely.