Make kas configurable (chart)

Release notes

Users of Kubernetes Agent Server (kas) until now had to use command line arguments to customize their kas installations. This setup proves inadequate for large scale installations where declarative, repeatable setups are a core part of the workflow and many arguments need custom settings. This change adds ConfigMap support to the Helm based Kubernetes Agent Server installation that provides the needed flexibility for large scale deployments.

Summary

In order to allow configuration of Kas

For example, configuration of Sentry, we will need to add these new values to the KAS sub-chart.

Current behavior

We cannot provide a config file for kas

Expected behavior

User has ability to set configuration for every setting in the config file for kas (see https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/blob/master/doc/kas_config_example.yaml for example)

Proposal

1. Have a minimal config for Kas, with hardcoded structure
2. Have a customConfig value that replaces the default config when given

See #2372 (comment 446593459) for reasoning

Backwards and forwards compat

Ideally a solution will be able to handle any changes to the kas config file format

Relates to gitlab-org/cluster-integration/gitlab-agent#27 (closed)