Emit JSON schema and BigQuery schema

[duncanjbrown]

When setting up a new project using dfe-analytics, it's necessary to create a schema in BigQuery that's compatible with the schema in this project.

BigQuery uses its own schema format, so we need to map the JSON schema to that format. The BigQuery schema contains less information than the JSON schema (e.g. it lacks enums, additionalProperties etc). We keep the JSON schema as the source of truth for testing, and generate a BigQuery schema from it.

This PR adds two new rake tasks: dfe:analytics:schema and dfe:analytics:big_query_schema. The BigQuery schema is suitable for copy-pasting into the BigQuery web interface. It could also be used to generate a table via the bq CLI tool.

Changes to the schema!

During this process I noticed that the event-schema.json was not the same as the BigQuery schema (for Apply, at least) and I amended it (relaxing requirements, so not breaking changes)

[misaka]

Suggested change

	class EventSchema
	module EventSchema

Why is this a class and not a module?

[misaka]

I can't for the life of me understand why Rubocop is singling out lib/dfe/analytics/event_schema.rb for not having top-level documentation. Other classes and modules don't have top-level docs. I event tried copying over the entire contents of load_entities.rb into event_schema.rb, and Rubocop complains about event_schema.rb and not load_entities.rb! 🤯

$ be rubocop lib/dfe/analytics/load_entities.rb lib/dfe/analytics/event_schema.rb
Inspecting 2 files
.C

Offenses:

lib/dfe/analytics/event_schema.rb:5:5: C: Style/Documentation: Missing top-level documentation comment for class DfE::Analytics::LoadEntities.
    class LoadEntities
    ^^^^^^^^^^^^^^^^^^

2 files inspected, 1 offense detected