Use the protoc build plugin in examples

[glbrntt]

Motivation:

We now have a SwiftPM build plugin to generate code as part of the build
process, the examples should make use of it.

Modifications:

• Remove generated code for examples and update the generate script to
  stop genearting example code.
• Symlink protos from examples to the checked out location in the repo;
  this avoids some duplication and makes it easier to keep things
  up-to-date.
• Add a basic config to each example.
• Update CI to install protoc before building examples.

Result:

Better examples, less code.

[gjcairo]

Nice one