Update examples to dynamically generate FileDescriptorSet

[Jacob Bandes-Storch]

Nowadays, rather than suggesting protoc --descriptor_set_out, we point people at a technique to generate the FileDescriptorSet at runtime, which is used in mcap libraries/examples:

https://github.com/foxglove/mcap/blob/21b737f99800bfe1d733b769022a726ab3ca4e0e/python/mcap-protobuf-support/mcap_protobuf/schema.py#L17

https://github.com/foxglove/mcap/blob/21b737f99800bfe1d733b769022a726ab3ca4e0e/cpp/examples/protobuf/writer.cpp#L48

We should update the examples in this repo to use the technique.