Refactor payment method buttons and separators rendering

[timur27]

Why is the refactoring needed?

There was a fix implemented here and here that resolves the issue of displaying multiple separators on the checkout, cart, product details pages.

After the fix, the button and separator displaying implementations are separated from each other. With that, a few drawbacks are currently visible, and these should be resolved once the refactoring is completed:

1. In a nutshell, the logic deciding whether we should render the button and the separator is duplicated (see WC_Payments::display_express_checkout_separator_if_necessary for the separator and WC_Payments_*_Button_Handler::init for Platform_Checkout, Payment_Request and Stripe_Link accordingly. This will lead to situations when e.g. the WooPay button logic is changed in the future, we might easily forget to update the logic in the separator rendering class, which will immediately result in displaying the separator alone when the button is disabled.
2. Separator rendering logic is located in class-wc-payments.php, which doesn't seem to be its direct responsibility.
3. There's limited test coverage due to static methods in the class-wc-payments.php class.

Ideally, we could create a new class that will merge and orchestrate all the logic from both button and separator rendering in one place and test it out properly.

Acceptance criteria

1. Button and separator displaying logic is merged into one place.
2. The bug with the multiple separators doesn't appear (no regression after the changes within this issue).
3. All the logic is covered with tests.

[jessy-p]

@Automattic/pulsar

[timur27]

Hey @zmaglica @jessy-p. Currently, this issue can be addressed within the split-UPE project only as the split UPE branch wasn't merged yet. After it's merged, this can be taken basically by any team to work on. Before this happened, I think adding category:projects label and possibly addressing this by @Automattic/heisenberg-woopay-upe-compatibility would be the best option. Sorry for not providing this information earlier.