[CI] Add client integration test as an optional PR job

We sometimes want to verify that the `miden-client`'s integration tests still pass against a PR.

This job should be opt-in only because

1. it's expensive to build and run both the node and integration tests, and
2. the client and node are not developed in lock-step so the tests aren't always conclusive evidence of bad node code

(1) is somewhat solvable using caching, but (2) not so much.

I propose enabling the job using a label, similar to how the changelog check operates (but in reverse). So adding the `client test` label to a PR would trigger and enable the integration test job.

The job would need to

- run the node locally,
- clone `miden-client` and `make integrate-test`

It possible that this is too much work for a generic github worker; we'll have to see how badly this runs.