Reactive support for transit operations

[cosminseceleanu]

Is there a reason why there is not a reactive version of VaultTransitOperations? are you planning to add this?

[mp911de]

With the initial reactive support, we wanted to learn how much the API will be actually used. Also, co-maintaining multiple implementations (reactive and imperative variants) for the same functionality imposes additional maintenance effort. In any case, we can upen up bits of the imperative transit support to make it easier for reactive reuse with ReactiveVaultTemplate.