[Feature]: Add list of checkouts

[witoszekdev]

Our customer should be able to see the list of unfinished checkouts and be able to filter out checkouts that have a payment status attached, but weren't completed by users for some reason.

Example use case:
Our customer has an issue in their storefront that causes some checkouts to be uncompleted (ie. not turned into an order) but their flow requires to firstly pay for the checkout and create an order later. In such case the payments are not automatically refunded and need to be done manually by the staff members - either create the order from the checkout or refund the Transaction

Example query:

query {
  checkouts(filter: {chargeStatus: [PARTIAL, FULL, OVERCHARGED]}) {
    # ...
  }
}