Implement isPending()

[judgej]

Some mPAY24 responses must be interpreted as pending. At the moment they are not caught.

According to the mpay24 documentation, there are those values possible:
https://docs.mpay24.com/docs/transaction-overview#transaction-state

• Capture : isSuccessful() -> true (for captures)
• Failed : isSuccessful(), isCancelled(), isPending() -> false
• Authorization : isSuccessful() -> true (for authorizations)
• Credit : Can there be an isRefund() method returning true for that case?
• Cancelled : isCancelled() -> true
• Redirect : isPending() -> true
• Pending : isPending() -> true
• Waiting for confirmation : isPending() -> true
• Chargeback : Maybe also isRefund()?
• Withdraw : isSuccessful() -> true

The \Omnipay\Common\Message\NotificationInterface defines STATUS_PENDING
as additional status for $omniRequest->getTransactionStatus() when
isSuccessful() returns true.

[judgej]

The main issue is that there is no one place to find a list of all transaction states. The table linked above is about a third of them, and the field shown is what is displayed in the administration screens and not what is sent in the API. For exampe Credit is sent as CREDITED. They are different values and could actually mean different things, possibly depending on the context.