Agent should gracefully terminate subprocesses in Windows - need a console

[pchila]

PR #7738 attempted to introduce graceful termination for elastic-agent subprocesses but it does not seem to be working (ref #7756 and #6875 (comment)).

On windows, the graceful termination of subprocesses is handled by sending a CTRL+BREAK event, using GenerateConsoleCtrlEvent function specifying the correct pid (which should really be the process group id).
In the documentation we can read:

Only those processes in the group that share the same console as the calling process receive the signal. In other words, if a process in the group creates a new console, that process does not receive the signal, nor do its descendants.

When elastic-agent is running as a windows service it does not get a console, hence it cannot signal child processes properly give that there's no console shared between the processes (in unit test everything works as the go test command has a console of its own that is inherited by subprocesses).

The goal of this issue is to fix the signaling on windows, ensuring that a console is shared between elastic-agent and its subprocesses and that it is used to signal subprocesses to shutdown.

Notes:

• A process can have at most one console attached (so it's a global object), some synchronization may be required when calling AllocConsole, AttachConsole or FreeConsole
• A new console may be allocated when invoking subprocess (as opposed to inheriting the caller's console) using process creation flags like CREATE_NEW_CONSOLE. Alternatively a new console can be created "on demand" using AllocConsole

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[pchila]

An example of setting up a console for a subprocess (an elastic-agent watcher in the specific case) can be found in PR #8767:

• Dynamically allocate a new console before spawning a subprocess and detaching right after
• Attaching to the target process console and signaling