Add breaking change detection

following up on slack conversation about possibility of using `buf` breaking change detection

the context was a discussion about moving Envoy's protos to the xds repo and whether we would be able to avoid running all of Envoy's CI in order to detect breaking changes

i dont have a concrete answer to that yet, im just getting a feel of how buf works, but i think probably the best first step is to add the buf dependency, and some initial linting (#45 )

from there i would suggest we look at if/how it could be used with existing xds protos

my initial (limted) understanding is that it works by checking local protos against published ones - it may not do what we need in terms of above goals - but is probably still quite useful even if it doesnt