Add a way to enable the beats receivers feature flag for package tests

[cmacknz]

• Relates https://github.com/elastic/elastic-agent/blob/main/docs/hybrid-agent-beats-receivers.md

As we continue the conversion of Beats based Elastic Agent inputs into Beat receivers in the OpenTelemetry collector, we will want to make use of the existing integration package tests to validate them. To simplify this we need an easy way to inject the beat receivers feature flags to the running configuration via elastic-package. A command line option is probably simplest.

How to enable Beats receivers for inputs is documented in https://github.com/elastic/elastic-agent/blob/main/docs/hybrid-agent-beats-receivers.md. The runtime (process or otel) can be specified directly on each input.

[cmacknz]

Right now beats receivers is only allowed for limited input types (mostly those used by agent self-monitoring). We would need to open this up to all input types which is easy enough to do.

It may be simplest to allow enabling this via an env var, for elastic packages use a variable that instructs agent to run everything that can be a beat receiver as a beat receiver would probably be enough and would insulate testing here from any further adjustments to the feature flag structure.

[cmacknz]

elastic/elastic-agent#10821 to track adding environment variables to control use of beats receivers.

[leehinman]

I think we need to delay this until https://github.com/elastic/ingest-dev/issues/5567 is done.

For the system tests, elastic-package first enrolls the agent with a policy and then adds each datastream. This means we can't use the override for the input because you first have to know the ID of the input before you can push an override. What the means is that you could start with the process runtime and then switch to the otel runtime. Which means for things like "logs" you could have consumed all the test logs before you switched to the otel runtime.