Introduce wmgUseMonologLogger feature flag

This will be used to control switching wikis from MWLoggerLegacySpi to
MWLoggerMonologSpi. Introducing in a separate commit so that it can be
synced well in advance of use to prevent "Undefined variable" warnings.

Bug: T76759
Change-Id: I61fa967ef376f6e9667f0028826f61ab15d94038