Add ability to deploy custom elastic-agents on different OS or runtimes

There are some integrations that might require elastic-agents with custom configurations for them or their container ie: winlogbeat requires a windows container, auditbeat requires special container capabilities, etc.

I initially created https://github.com/elastic/elastic-package/pull/786 that adds ability to deploy custom agents as test services, there is still missing code specific to deal with the windows scenario.

I open this thread to discuss other approaches that might avoid adding the complexity to the test runner if possible.

**EDIT:**

As mentioned in https://github.com/elastic/elastic-package/issues/787#issuecomment-2180368953 , there is now support in `elastic-package` to:
- run tests in independent Elastic Agents (for now in Linux):
- these independent Elastic Agents can be customized (with capabilities and scripts).
- each system test writes into its own data streams
- run system tests in parallel
- being able to set through an environment variable the maximum number of routines to run in parallel

It will be pending here to allow running Elastic Agents in other OS (e.g. Windows) or in other runtimes (VMs?).