Add proto files

Closes #1 #2

Description

This MR contains changes for both #1 and #2. It's heavily inspired by the prototype work done by @mk2155 in https://gitlab.developers.cam.ac.uk/uis/devops/experiments/mk2155/protobuf-prototype. There are a couple of differences though:

• Using betterproto (buf.build/community/danielgtaylor-betterproto:v1.2.5) instead of buf.build/protocolbuffers/python and buf.build/protocolbuffers/pyi
• Change the folder structure to also version AccountSelectorEvent because buf-lint was complaining that the structure in the prototype was not correct

This MR also adds:

• pre-commit with buf-lint and buf-format
• GitLab pipeline to run pre-commit
• Information how to generate the Python files locally
• release-it to handle versions