APIv4 - Cache SchemaMap in Civi::cache('metadata')

APIv4 - Cache SchemaMap in Civi::cache('metadata')

When adding/removing custom groups, the schema map changes.
Civi already flushes the metadata cache when this happens,
but the APIv4 SchemaMap was being cached in memory (in a container service).
This changes it to be stored in the metadata cache like everything else.