[prometheus-kafka-exporter] Replace bitnami's kafka chart in CI, support extra manifests

[zeritti]

What this PR does / why we need it

Since Bitnami's images referenced in the dependency Helm charts will not be available as it has been so far from the end of August onwards (Reference), the dependency chart being used in CI needs to be replaced. Unlike in other similar cases (Nginx exporter, PgBouncer exporter), Kafka exporter does not start without connecting to the target service, a Kafka broker, at all as it cannot init a Kafka client - ct install would always fail. In this PR I am proposing to use Strimzi Kafka Operator in CI as a replacement of the Bitnami's Kafka chart. Strimzi is an incubating project at CNCF with images available from quay.io.

Summary of the changes:

• Use Strimzi Kafka Operator and its CRs in CI - a single Kafka broker gets created for the exporter to connect to
• Add further configuration in ci/ci-values.yaml
• Add support for extraManifests
• Add a hint for renovate
• Enter correct sources in Chart.yaml

Which issue this PR fixes

None

Special notes for your reviewer

Checklist

• [x] DCO signed
• [x] Chart Version bumped
• [x] Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])