Reshape APIs

[rwinch]

Currently the APIs of VaultTemplate and VaultClient look a lot like RestTemplate. If users want to do the REST operations, they already have RestTemplate to do this. It would be nice if the VaultClient APIs reflected the intent rather than the underlying REST semantics.

[mp911de]

This change should reshape VaultSession as well, VaultSession continues the approach from VaultClient.

[mp911de]

There are two issues in this ticket:

1. After discussing with @rwinch we found out that Rob wasn't aware of VaultOperations.
2. Spring Vault defines a RestTemplate-like through VaultClient. It has a huge API surface and is somewhat an abstraction over RestTemplate with little benefit.

I created two PR's to evaluate different approaches (functional VaultClient exposing less API surface, removal of VaultClient).