increase succintness of Future handling

[tomholub]

I'll file an issue to create a slightly different convention here for increased clarity.

Originally posted by @tomholub in #400 (comment)

[tomholub]

[tomholub]

Here I was thinking it would be great if the type could be extended, so that instead of:

        messageSender
            .sendMail(mime: encrypted.mimeEncoded)
            .sink(
                receiveCompletion: { [weak self] result in
                    guard let error = result.getError() else {
                        return
                    }

                    self?.showAlert(error: error, message: "compose_error".localized)
                },
                receiveValue: { [weak self] in
                    self?.handleSuccessfullySentMessage()
                })
            .store(in: &cancellable)

We would have:

        messageSender
            .sendMail(mime: encrypted.mimeEncoded)
            .sinkFuture(
                receiveValue: { [weak self] in
                    self?.handleSuccessfullySentMessage()
                },
                receiveError: { [weak self] error in
                    self?.showAlert(error: error, message: "compose_error".localized)
                }
            )
            .store(in: &cancellable)

or

        messageSender
            .sendMail(mime: encrypted.mimeEncoded)
            .sink(
                receiveValue: { [weak self] in
                    self?.handleSuccessfullySentMessage()
                }
                receiveError: { [weak self] error in
                    self?.showAlert(error: error, message: "compose_error".localized)
                },
             )
            .store(in: &cancellable)

Because I imagine we will be seeing a lot of code like this, it would be great if it could be streamlined as above.