**What this PR does / why we need it**:

This PR adds the Loki canary as well as the GEL provisioner to the Loki
helm chart. The Loki canary expands the self-monitoring functionality of
the Helm chart, giving operators insight beyond whether Loki is just
running, but also if it's functioning properly.

The GEL provisioner was necessary for the canary to work when the
enterprise option (`enterprise.enabled = true`) is enabled. Since
enabling enterprise in the helm chart also enables auth/multi-tenancy,
the addition of the provisioner allows the helm chart to automatically
create a tenant for the canary via the GEL admin API, and to create
applicable access policies and tokens. Furthermore, additional tenants
can also be specified, which will be provisioned, creating a read and
write token for each that will be stored in k8s secrets.

**Which issue(s) this PR fixes**:
Fixes #7019 

Co-authored-by: Karsten Jeschkies <[email protected]>
Co-authored-by: Dylan Guedes <[email protected]>