expose custom clock setting?

[immerhart]

It would be useful to be able to handover a custom clock, if used in e.g. DES or some other runtime. I found a mock clock in the tests, which basically does this already:
https://github.com/moka-rs/mini-moka/blob/v0.11.x/src/unsync/cache.rs#L1245
and set_expiration_clock.
Is there any reason its not exposed?