Add a GraphQL type for the audit event definitions

We now have audit event type YMLs that are defined under
[`ee/config/audit_events/types/*`][0] and
[`config/audit_events/types/*`][1]. We want to expose these type
definitions in the GraphQL API.

This change adds a GraphQL type for the audit event definitions along
with exposing the yaml definitions via and added query resolver

Rubocop required a policy to be added, however this data is already
public via the codebase, so I wasn't sure what to do there. Open to
suggestions

[0]:https://gitlab.com/gitlab-org/gitlab/-/tree/master/ee/config/audit_events/types
[1]:https://gitlab.com/gitlab-org/gitlab/-/tree/master/config/audit_events/types

Changelog: added