API for loop configuration options

This issue doesn't propose a specific codebase change but is a point for discussing future steps in the plugin evolution.

Python has deprecated event loop policies since (unreleased) py-3.14.

The main asyncio motion idea for this and related changes is:

1. Don't instantiate the loop explicitly, but use Runner (introduced in 3.11, there is a backport library for older Python version).
2. Control runner behavior by debug and loop_factory arguments.
3. There are eager tasks since 3.12; maybe the runner will have the option to configure it as well if we make an agreement

I think pytest-asyncio should reflect these changes.

1. event_loop_policy() fixture should be deprecated along with event_loop.
2. The library should not introduce public fixtures except unused_tcp_port and family. It is very fragile, especially if a user starts fixture overriding.
3. Fine-grained control could be done by special pytest.mark.asyncio(...) arguments.
4. Coarse-grained control uses configfile parameters and, maybe, cmdline args.
5. Implementation details could vary; we are free to change them in the wild until the details are not leaked into the public space. The plugin already has a good movement in this direction, so I merely summarized what we do.

Please share your opinions in the comments, guys.

Start by reviewing the event_loop_policy() and event_loop fixtures, pytest.mark.asyncio arguments, and configuration or command-line behavior described in the issue. The issue is a discussion of future API direction rather than a defined implementation. Done would require agreement on the public configuration design and the resulting implementation scope.