Add bdld::DatumMapOwningKeysBuilder::allocator() method
Add bdld::DatumMapOwningKeysBuilder::allocator() method

This fixes API compatibility with dlct::DatumMapOwningKeysBuilder.

DRQS 87536751

bloomberg/bde,bloomberg/bde,bloomberg/bde,che2/bde,che2/bde,che2/bde,bloomberg/bde,bloomberg/bde,che2/bde