Add secrets engines abstraction

[damian-kwolek]

Hello,

I'm suggesting adding another layer of abstraction based on Vault's own API docs - Secrets Engines (overlays).
They can be used to streamline path generation for specific jobs and objectify whole Vault transaction.
For example, in case of KeyValue Version 2:

• You won't longer have to remember which http method to use and which "keyword" to add in between mount and secret path
• Configuration, metadata, write options, etc. can now be standarized objects instead of associative arrays, so it's more clear what you can or cannot set
• Responses can now be more structurized (using builders), because such overlay can expect certain response structure from Vault

This PR:

• adds support for KV1 and KV2 with free Cubbyhole (as it is KV1 at predefined mount) secret engines
• fixes few small typos
• corrects Content-Type for PATCH method, as simple application/json may be rejected by newer Vault versions

I made sure it does NOT break backwards compability, so you can still use old way of raw calling client directly.

[CSharpRU]

Needs an interface for all secret engines :)