Allow setting Elastic Agent version independently from stack version #2852
Description
It is possible to set the stack version by setting the --version flag. By default, this also sets the Elastic Agent version. A custom Docker image can be specified using environment variables or the configuration file, allowing a different agent version. As a new agent.version condition is introduced in package manifests in order to specify the minimum compatible version of Elastic Agent version, we should introduce an --agent-version flag as a quicker way to set the agent version. If the agent version is overridden using the ELASTIC_AGENT_IMAGE_REF_OVERRIDE environment variable or stack.image_ref_overrides.elastic-agent configuration, these should override the flag.
In addition, it should be possible to run system tests with variable agent versions, by default with the oldest supported version, as we do in other scenarios to avoid breaking compatibility with old versions.