new virtualenv handlers

[memsharded]

Changelog: Feature: New VirtualRunEnv handler to do custom copy and env-var definition.
Docs: https://github.com/conan-io/docs/pull/XXXX

Close #18627

This is just a preliminary proof of concept to discuss with the team.
Some points:

• It seems that Conan could provide some built-in default handler, similar to the runtime_deploy
• The overlap with runtime_deploy and the feedback over it so far, with different users wanting different things from it, is a concern, this is why I am proposing a customizable behavior, injecting a handler, not just some arguments
• It should be extended to VirtualBuildEnv too

TODO: Check if the deployers strategy could adapt the VirtualRunEnv paths a posteriori, as CMakeDeps is doing with the full_deploy