Move kube-stack-mixins into tenant observability

We configure kube-prometheus-stack through a jsonnet mixin mechanism:

• https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/instrumentor/blob/fe446e5f3fff0b659673b3de468c0fe19e729317/common/jsonnet/kube-stack-chart-mixins
• https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/instrumentor/blob/cdcbd3c7d92cd0c6a5181d56da551924df27d147/aws/jsonnet/kube-stack-chart-mixins
• https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/instrumentor/blob/cdcbd3c7d92cd0c6a5181d56da551924df27d147/gcp/jsonnet/kube-stack-chart-mixins

This generates input values we use in kube-prometheus-stack (example).

This configuration should be moved into tenant-observability instead of making the entire JSON document an input for the stack. We will expose necessary parameters in tenant-observability-stack and based on that generate the configuration instead if having it passed in.

Using jsonnet-provider may be an option here.