Allow override image of individual components in Helm chart

[narqo]

Right now the mimir-distributed Helm chart only has a global configuration, that defines the container image, all Mimir components use (ref image and enterprise.image in the default values).

To be on par with Mimir Jsonnet bundle, the chart must provide a way to override image for an individual component (e.g. ingester, querier, store-gateway, etc).

Multiple use-cases for that are coming from the Grafana's internal experience:

1. When backporting a hotfix into a weekly release, we may want to avoid re-deploying the whole system, and only (temporary) update one of the components
2. Similarly, during testing, only a subset of components (usually single component under testing) may need to be overridden with a private development copy

[narqo]

Examples from other database charts:

• Loki
• Tempo