Migrate elixir from protobuf to JSON Schema based messages

[aurelien-reeves]

Ruby, JS, JVM and other implementation have migrated from protobuf to JSON Schema based messages.

Refs. cucumber/common#1386

[aurelien-reeves]

@WannesFransen1994 I guess you already started working on that. Did you already push some code to the server? Is it possible to open a PR?

[WannesFransen1994]

Sorry for the delay, the PR is cucumber/common#1952. I'll be working on it in the upcoming weeks.

[mattwynne]

See #29