[messages - elixir] Convert proto-based messages to json schema based messages

[WannesFransen1994]

Summary

Remove protobuf dependency and create modules based on json schema (manual parser). No validation of data in messages, that's done through acceptance testing.

Checklist:

• [x] Clean dependencies
• [x] Generate modules in generated folder
• [x] Adjust Make file & do integrations in CICD
• [x] See if compatibility testing kit is relevant
• [x] Adjust gherkin library as well and verify with acceptance testing there
• [x] Provide meaningful debug messages if applicable

[WannesFransen1994]

Attempted a makefile adjustment, but seems that there's a problem with the messages generation. (based on the log)

==> cucumber_messages
Compiling 3 files (.ex)
Generated cucumber_messages app

[WannesFransen1994]

Not that familiar with the whole monorepo build, but I've managed to make it work with adjusting the Makefile. Would love some pointers / reviews on the integration, specifically for

1. manually copying the json schema files
2. instead of adjusting the general gitignore - which caused an rsync conflict-, I've adjusted an ignore file here
3. The CircleCI config integration file
4. Instead of running the compiled binary with every file and verifying the output, I do this in the test suite (since starting an beam VM every time is expensive). Saves a lot of time though (would be even more if we ran this in parallel, though not sure it is worth the effort)

[aurelien-reeves]

PR now available in the dedicated repo here: https://github.com/cucumber/messages/pull/29