We have a few different .proto files in this repository that all need to
get recompiled into .pb.go files each time we change them, but we were
previously handling that with some scripts that just assumed that protoc
and the relevant plugins were already installed on the system somewhere,
at the right versions.

In practice we've been constantly flopping between different versions of
these tools due to folks having different versions installed in their
development environments. In particular, the state of the .pb.go files
in the prior commit wasn't reproducible by any single version of the tools
because they've all slightly diverged from one another.

In the interests of being more consistent here and avoiding accidental
inconsistencies, we'll now centralize the protocol buffer compile steps
all into a single tool that knows how to fetch and install the expected
versions of the various tools we need and then run those tools with the
right options to get a stable result.

If we want to upgrade to either a newer protoc or a newer protoc-gen-go
in future then we'll do that in a central location and update all of the
.pb.go files at the same time, so that we're always consistently tracking
the same version of protocol buffers everywhere.

While doing this I attempted to keep as close as possible to the toolchain
we'd most recently used, but since they were not consistent with each
other they've now all changed which version numbers they record at minimum,
and the planproto stub in particular now also has a slightly different
descriptor serialization but is otherwise offering the same API.