Support for multiple consumer schemas within a topic

[untitaker]

Currently, each topic has exactly one latest version which can only have one schema.

We already run into limitations with this design as per #80, because we want to have some sort of type definition for some usecase unrelated to the needs of the consumer that the schema was designed for. As a result we end up adding restrictions to the schema that go far beyond the consumer's own needs.

This might become a problem because in a world where validation errors become more than warnings, it makes the consumer more brittle and reject/DLQ messages that it would've been able to process fine.

Proposal is to add a concept of consumer groups to sentry-kafka-schemas, and then each consumer group is individually versioned (instead of the topic). As a result we can have an arbitrary amount of "latest" schemas for a topic, and even run validation for all of them in the snuba consumer, but only one schema's validation errors cause DLQ/rejection of messages. The rest can be warnings.

For documentation purposes, a combined view of all schemas can be rendered, or all schemas can even be combined into one using allOf if that is somehow useful.