[mlgo][nfc] Virtualize Logger implementation

This is in preparation for dropping the dependency on protobuf. This
first step allows us to subsequently introduce the non-protobuf
implementation behind a flag. After that we can update the training side
to ingest the new format, after which we can drop the protobuf
implementation and de-virtualize everything.

Differential Revision: https://reviews.llvm.org/D139062