add an option to prefix names of generated client/server types

[propan]

Hi!

We use proto-generators extensively in our code base and the generated types collide with some existing types, it would be convenient to have an option to prefix generated client/server type name like:

option (nrpc.typesPrefix) = "NRPC";

With these changes, the default behaviour stays the same.

It would be great to get it merged!

Thanks.

[cdevienne]

This is a nice addition to nrpc, thanks!

To be merged, we need a demonstration in an example (can be alloptions), and since I just updated the dependencies in the master branch, a rebase keeping only the new feature.

Also, I think it is not prefixing all the types, at least the "xxxHandler" type is not.

Thanks for contributing