[Sign in with site credentials] - Don't clear username and password after an invalid login attempt.

[selanthiraiyan]

For woocommerce/woocommerce-ios#7444

Description

In the sign-in with site credentials flow, in the Site credentials screen, if I enter an incorrect password, the username and password fields are emptied/cleared. This doesn't leave a chance for the user to edit and fix types.

This PR attempts to stop clearing the username and password when an error occurs.

• The username/password is cleared when the tableView is reloaded after a failed signin attempt. We can configure the cells with the current username/password from loginFields property to fix this.
• We have also updated the prepareForReuse method to properly reset all the properties that are customised here. Not preparing the cell for reusue properly has introduced issues like [Sign in with store credentials] - Username field turning into "secret field" woocommerce/woocommerce-ios#7403 in the past.

Testing steps

Please refer to the relating WCiOS PR to test the integration.

[itsmeichigo]

LGTM 🚀