Feature Overview

Enable configuration in Backstage CR for mounting files and injecting environment variables into non-backend containers.

Goals (aka. expected user outcomes)

There is a needs in configuring mounting files and inject env variables into containers other than backstage-backend in the Backstage CR.
E g https://issues.redhat.com/browse/RHDHBUGS-887

The places to apply new configuration include:

• spec.application.extraFiles.configMaps
• spec.application.extraFiles.secrets
• spec.application.extraFiles.pvcs
• spec.application.extraEnvs.configMaps
• spec.application.extraEnvs.secrets
• spec.application.extraEnvs.envs

Requirements (aka. Acceptance Criteria):

The feature provides:

• A way to easily configure files and environment variables to be injected into the default `backstage-backend` container (no additional configuration needed), as well as into a list of containers by name (including `dynamic-plugin-install` and sidecars) or all containers in the Backstage Pod.
• Related upstream and downstream documentation.
• A comprehensive set of tests.

The feature does not introduce any breaking changes.

Documentation Considerations

The feature should be properly documented upstream and downstream.