Update event-type schemas to be stricter

[svix-gabriel]

This updates the /event-type endpoints "schemas" parsing to match our API more closely.

Motivation

In our API, we specifically parse event-type schemas as a map of version numbers to Json Schemas. While we don't enforce the version numbers, we do enforce that the Json schemas are, well, valid JSON schemas!

The open-source repo was allowing arbitrary JSON objects, which is too lax.

Solution

We use the jsonschemas library to parse Json Schemas. This library has a compatible license (MIT), no known vulnerabilities, is actively maintained, with modest popularity (283 stars in Github, a little over half a million downloads).

We still need to (de)serialize the event_type::Schema, and jsonschema::JSONSchema doesn't implement any of the (de)serialization traits. Consequently, the type definition is still a wrapper over a Hash of strings to Json, and we manually implement deserialize to enforce that that the json schema is always correct.

Oh wait, can you provide some information about the new crate?

[svix-dylan]

Almost forgot, we are supposed to provide some information about open source libraries when we add them to deps. Here are some examples.

[svix-gabriel]

Thanks @svix-dylan, I updated the PR message with info on the jsonschema crate.