Make <db>.system.stats.ndv a legal system namespace and define the persisted document format as strict IDL shared by the write and read paths. Follows the persistent samples precedent (system.stats.samples, persistent_sample.idl).

• src/mongo/db/namespace_string.h: add kStatsNdvCollectionName + isStatsNdvCollection()& whitelist in isLegalClientSystemNS() (namespace_string.cpp)
• New ndv_stats.idl: NdvSketch