Start using protolint

It's often the case that we need to ask for comments to be added when proto files are modified. I think we could save a bit of back and forth by introducing a much more strict proto linter.

This MR introduces https://github.com/yoheimuta/protolint

Comment linting is still turned off here because of the large number of existing offences, but the comment linters are broken down into a few different types, so hopefully it would be possible to do this in stages:

• MESSAGES_HAVE_COMMENT
• SERVICES_HAVE_COMMENT
• RPCS_HAVE_COMMENT
• FIELDS_HAVE_COMMENT
• ENUMS_HAVE_COMMENT
• ENUM_FIELDS_HAVE_COMMENT
• FILE_HAS_COMMENT

There should be no changes to the RPC in this MR, although adding no-lint comments does change the raw definition.