`Reusable` and `NonReusable` reverse mode capability

[mzgubic]

From discussion on Slack with @chengchingwen

The idea is to define a new capabilities Reusable which means that the pullback can be called multiple times, and NonReusable, which means that the pullback can only be called once.

NonReusable can be useful when wanting to save allocations, such as in JuliaDiff/ChainRulesTestUtils.jl#264