Add standard getCreateToken/setCreateToken request methods

[leith]

Most payment processors seem to have a "please store my card details" feature, where they pass back a token to the merchant to represent the card. We already have standard getToken() and getCardReference() methods for this parameter if it exists to pass along instead of card details, but no standard method for indicating that the token should be created as a part of the request in the first place.

Usually this means it ends up as an implementation detail with whatever the parameter is called (tokenize, tokenMode, storeCard, etc.) and is one more detail that must be accounted for per-processor rather than something that can be relied on at a higher level to be a consistent flag.

This PR standardizes that flag as getCreateToken() and setCreateToken() (the same as https://github.com/thephpleague/omnipay-sagepay/) so we can reliably set it once and know the driver will convert it appropriately.

[leith]

The CI failure message is:

No filter is configured, code coverage will not be processed

This seems to be unrelated to the branch - higher versions of PHP (8.1+) are using newer versions of PHPUnit, which have a different XML configuration format. I've made a separate PR (#276) to fix the CI pipeline in case that's needed before this can be evaluated.

[leith]

With #276 now merged, CI is running and all passing as well. 👍

[leith]

@judgej since this is largely based on your work on the sagepay driver, what do you think about making this a common flag?