feat: allow sending generic API requests

[MaximBalaganskiy]

This will allow for calling APIs which are missing in the current implementation

[Zetanova]

would resolve #342

[rajanadar]

this is a fantastic intent @MaximBalaganskiy . I 100% understand what this is meant to do.

However, the sole purpose of VaultSharp is to be that strong first-class C# layer to hide all http/api things.

It is also the motivation for users to raise missing api features and for us to add it as strongly typed methods.

Doing this will open up the http world to callers, who will then have this weird hybrid code in production and management rarely gives time to unwind those things.

I am going to close this PR solely due to the core tenet of the library. Not to expose polymath/http at all.
Your intent is 100% appreciated.