Allow ebpfcore to be used as a user-mode platform

### Describe the feature you'd like supported

For example, to allow extending a system service.
This means the user-mode platform would be usable in production, not just for testing.
Usersim would be used for the kernel-mode platform.

### Proposed solution

There would thus be 3 ways to execute tests:
1) Kernel mode natively (some test items not supported, like time travel debugging, code coverage, and/or fuzz testing)
2) Usersim compilation of the "kernel-mode" platform
3) Runtime compilation of the user-mode platform

### Additional context

_No response_

No files or tests are named. Start by locating the kernel-mode platform, Usersim compilation path, and user-mode platform test execution entry points; define how the three proposed execution modes are selected and validated. Done means the user-mode platform can run production system services while Usersim remains available for kernel-mode execution.

Written by the indexing model from the issue text.