Prefixed namespaces

[izolight]

This MR adds the option to prefix namespaces to generated types.
This is necessary if you have types in multiple namespaces with the same names or having fields with the same name.

[liff]

IIRC prost generates modules for the namespaces, so that you get protos::org::example::namespace::Message. Do you think that’d work here as well?

[izolight]

I am already trying to get the module approach working in a separate branch.
The prefixed was just much easier to get working as my initial goal was to get rid of name conflicts. But seeing how impractical the names get with bigger schemas I might consider withdrawing this once I get the namespace as module working.

[flxo]

@izolight thanks for investigation! any updates on the module approach?