Add an in-memory file system API, which would be an extension of WT_FILESYSTEM. It should be backed by shared memory so that its state can be preserved across child process crashes and restarts, and if possible, it should be possible to also write the file system contents to files for debugging.

Having an in-memory file system should speed up workload execution, and it should also give us the ability to take efficient snapshots during test execution, so that we can "go back" to different file system states during the test. This would be useful if we decide to pursue deterministic state space and workload space exploration as suggested in the Metis paper from FAST '24.

However, as for now, this is a low-priority item.