Make Lazy caches support get_tmp

[LilithHafner]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[LilithHafner]

Motivated by DARPA-ASKEM/sciml-service#141, this facilitates seamlessly switching from DiffCache to LazyBufferCache without changing get_tmp calls to getindex calls. I'm not sure what the point of the get_tmp(dc, u) = dc method is, and someone who does know why that method exists should confirm that providing a more specific method here does not break anything.