Pull some widely-used value types under mongo/db into a base library

There are a set of common widely-used value types that 99% of the binaries under mongo/db need to reference. We should put them under a mongo/db/base library, similar to how mongo/base works.

Here are some initial candidates of .cpp files that need to go in there:

tenant_id.cpp

database_name.cpp

namespace_string.cpp

shard_id.cpp