Add support for in-config helm values

Release notes

https://docs.gitlab.com/ee/user/clusters/agent/gitops/helm.html

We shipped initial Helm-chart support for the agent for Kubernetes in GitLab 15.4. The initial solution required Helm values to be included as part of the Helm charts, leading to large-scale code duplication for our users.

The current release allows you to specify the Helm values as part of the agent configuration. This way, you can add environment-specific values to the agent configuration file and keep generic values within the Helm chart.

Problem to solve

It should be possible to set the values for a Helm release directly from the agent configuration file.

Background

See gitlab-org/cluster-integration/gitlab-agent!727 (comment 1078812703).

Proposal

Support inline value objects, under values.inline for a given release. For example:

gitops:
  charts:
  - release_name: my-release
    source:
      project:
        id: my-group/my-charts
        path: charts/the-chart
    values:
      - inline:
          tag: v15.5.1
          rbac:
            create: true

When values.inline is set, this should be deep-merged on top of (overwrite) the values from the chart.