remove necessary buf installation

[mpldr]

This commit adds buf as a regular dependency and go running it instead
of requiring installing buf to one's own GOBIN.

• read the CONTRIBUTING guidelines
• raised a GitHub issue or discussed it on the projects chat beforehand
• added unit tests
• added integration tests
• updated documentation if needed
• updated CHANGELOG.md

This also has the added advantage that every user is always using the same version.

[kradalby]

I was slightly mixed on this, but changed my mind when I realise this is what I set up for the actual gRPC plugins.

Unless working on API stuff, it isnt needed for building, but might as well have it available.

I am happy to add this now, but I will consider it obsoleted by #482, which defines the whole build env in a stricter manner.