Setup Intent implementation & off_session payments for Payment Intent

[sash]

We had to extend the plugin a bit to fit our needs at Ampeco, and I'd like to offer it back to the community.

The implementation of Session Intent does not cover the full api - just the Create and Retrieve methods are implemented. The rest can be easily added afterwards!

We also needed the off_session option in payments intent - that why we needed the SetupIntent in the first place :)

[barryvdh]

What does this do?

[aimeoscom]

@barryvdh It enables token based payments. We created a pull request (#178) which does roughly the same but retrieves the token after a regular payment instead of using a separate SetupIntent request

[sash]

Correct. The main difference being that using the setup intent there is no charge occurring the you only want to setup a credit card. That happens to be our use case

[hiqsol]

We need this too.
Will this PR be taken?

[barryvdh]

So now #178 is merged, is this still required / relevant?

[aimeos]

@barryvdh Yes, it's similar but doesn't perform an initial payment like #178 does

[sharald]

This PR and #178 both adds the method AuthorizeRequest::setOffSession(). The build currently fails, so this needs to be fixed.

[tafid]

This PR and #178 both adds the method AuthorizeRequest::setOffSession(). The build currently fails, so this needs to be fixed.

I have fixed it in #180