Support dynamic Kubernetes secrets

[julienduchesne]

Secrets are currently supported by being mounted on the load tester: https://github.com/grafana/flagger-k6-webhook#injecting-secrets-and-configuration

In cases where tests are coming from multiple namespaces/workloads, this can be a bit cumbersome because all secrets must be mounted on that central load tester deployment. The load tester is also restarted every time a secret is added (potentially cancelling some load tests)

The Flagger webhook payload should support a secret mapping structure that allows env variables to be created dynamically from Kubernetes secrets and injected into the k6 run process