Add HTTP MultiPart Post support

[hypest]

Adds HTTP MultiPart Post support to RestRequest in order to be able to post/upload files to endpoints. This enhances RestRequest, keeping its oAuth2 flow as well.

Changes:

• Enhance RestRequest by introducing a MultiPart "mode". When instantiated with a byte array, the RestRequest instance will use the array as its body
• Introduce a MultiPart Post request builder to construct a suitable RestRequest
• Add Stefanos to the developers list

Needs review: @maxme or @daniloercoli ? Thanks!