Ability to add custom authentication type

[adhithya-shankar]

My company uses a custom authentication, where signatures are generated using a timestamp and few other parameters signed using a private key.
And all the parameters and the signature needs to be sent part of the request as headers.

This is achievable via pre-request scripts.

But, it will be good to support plugin creation for custom authentication types. And this can appear under the Auth Types section of a request.