Enable optionally requiring postal code in CardInputWidget and CardMultilineWidget

[mshafrir-stripe]

Summary

By default, postal code is an optional field in CardInputWidget
and CardMultilineWidget because some countries do not have
postal codes.

For apps that are certain their customers are from a country
that has a postal code (e.g. US), postal code can be set to a
required field. If required, the postal code value must be
non-blank. No other validation is applied.

If the postal code field is not enabled (i.e. it is not visible),
then the postal code will never be required.

Note that because some countries do not have postal codes, requiring
a postal code will prevent users with cards from those countries
from submitting the form successfully.

Postal code can either be required via XML:

<com.stripe.android.view.CardInputWidget
    app:shouldRequirePostalCode="true" />

<com.stripe.android.view.CardMultilineWidget
    app:shouldRequirePostalCode="true" />

or code:

val cardWidget: CardInlineWidget = ...
cardWidget.postalCodeRequired = true

val cardWidget: CardMultilineWidget = ...
cardWidget.postalCodeRequired = true

Motivation

Fixes #2111

Testing

• Added unit tests
• Tested in example app
• Tested with TalkBack