Add FinancialMutation::linkToBooking()

[holtkamp]

As described here: https://developer.moneybird.com/api/financial_mutations/#patch_financial_mutations_id_link_booking

[holtkamp]

@stephangroen I had a look at the current approach to testing, it seems no mocking of the Moneybird API is done? Not that I would know how to do it 😉, I had some encounters with it in this project, any ideas / plans on how to mock the Moneybird API? (thinking of a MockingBird... 🐦 )

UPDATE
I gave it a shot and some initial testing using a mocked connection as explained here is added.

[holtkamp]

@stephangroen anything missing to merge this?

[stephangroen]

@holtkamp Looks quite nice! Currently no mocking of the API is done, only the verification if the entities are instantiable, so no code errors/typos exist. Your solution is a very nice addition for completer testing. Thanks!