Backport of Events API uses consistent JSON serialization and error codes into release/1.13.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #19246 to be assessed for backporting due to the inclusion of the label backport/1.13.x.

The below text is copied from the body of the original PR.

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A couple of small tweaks for consistency:

• UseProtoNames when serializing JSON fields to ensure we use underscores instead of proto3's default camel case to make us consistent with the rest of the Vault API. EDIT: Superseded by events: WS protobuf messages should be binary #19232 which changed to returning the cloud events formatted version of the event. I kept the additional test coverage though.
• Return 403 for unauthorized requests

For illustration on the status codes:

$ curl --fail -H "X-Vault-Token: $(vault token create -policy=default -field=token)" http://127.0.0.1:8200/v1/sys/mounts
curl: (22) The requested URL returned error: 403

$ curl --fail http://127.0.0.1:8200/v1/sys/mounts
curl: (22) The requested URL returned error: 403

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Overview of commits

• 4c11d09