Add two new environment variables to the elastic-agent container command:

1. ENABLE_BEATS_RECEIVERS as a boolean that when true uses Beats receivers for everything in the policy that can support Beats receivers. This should functionally address Add a way to enable the beats receivers feature flag for package tests elastic-package#2975 once elastic-package can support it.
2. ENABLE_BEATS_RECEIVERS_MONITORING as a boolean that is a short cut for setting the _monitoring_experiemental: otel setting in the agent.monitoring configuration, controlling whether beats receivers are used for self-monitoring or not.
3. Update https://github.com/elastic/elastic-agent/blob/main/docs/hybrid-agent-beats-receivers.md to document the two new variables for the container command.