Split out APIs used by C# codegen into separate namespace

[RReverser]

As the set of higher-level APIs grew organically, SpacetimeDB.Module namespace and SpacetimeDB.Runtime class became a mix of both low-level APIs used by automatically generated code and high-level APIs like Log meant to be used directly by the user.

To avoid a mess in autocompletions, it’s worth splitting out lower-level APIs used by codegen into a separate namespace.

Tyler note Feb 13 2024: We need to do this before 1.0 if we'd like stable APIs