Update credentials validation logic

[ristokekovic]

What is the goal?

• Update credentials validation logic by switching to the new dedicated endpoint for connection validation on the SeQura API instead of the merchant/{merchantId}/payment_methods endpoint that was used until now.

How is it being implemented?

• Update the SeQura\Core\BusinessLogic\Domain\Connection\Models\ValidateConnectionRequest with the new request parameters
• Update unit tests to reflect those changes

How is it tested?

• Unit tests
• Integration tests

[m1k3lm]

LGTM