Dump the Arrow registry itself as part of the generated code

[teh-cmc]

We build up the Arrow registry as part of the codegen process in order to be able to resolve datatypes.

The next step is to quote it directly into the generated code so that we can run meta queries at runtime (e.g. register.get_datatype("rerun.components.Point2D")?.

[teh-cmc]

Build-time registries don't really make sense in our brand new dynamic world where one can just send arbitrary arrow data anyway (also opens the door to all kind of nasty problems e.g. different schema versions across clients & viewer etc).

Runtime schema registries as well as deduplication still very much makes sense though, see:

• Implement minimal datatype registry for cross-batch deduplication #1809

Once this is in place, adding extra runtime-introspection APIs on top is trivial.