Clear Mooncake internal caches

[yebai]

As discussed in https://discourse.julialang.org/t/large-memory-consumption-when-using-mooncake-via-differentiationinterface-for-gaussian-process-optimisation/127289/8?u=willtebbutt , there's not currently a good way to clear the cached OpaqueClosures that Mooncake keeps around. We should probably add a function to the interface which promises to completely clear out any cached data that Mooncake currently holds on to.

[yebai]

It would be nice to avoid a new public API for emptying the cache.

How about passing a flag Config(;empty_cache=false) to prepare_gradient/derivative/hvp_cache functions? Under the hood, these functions would always empty the cache when empty_cache is true.

cc @sunxd3 @AstitvaAggarwal