This checks the request against the `read` permission for
`sys/events/subscribe/{eventType}` on the initial subscribe.
Future work includes moving this to its own verb (`subscribe`)
and periodically rechecking the request.
Tested locally by minting a token with the wrong permissions
and verifying that they are rejected as expected, and that
they work if the policy is adjusted to `sys/event/subscribe/*`
(or the specific topic name) with `read` permissions.
I had to change the `core.checkToken()` to be publicly accessible,
as it seems like the easiest way to check the token on the
`logical.Request` against all relevant policies, but without
going into all of the complex logic further in `handleLogical()`.
Co-authored-by: Tom Proctor <tomhjp@users.noreply.github.com>
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