feat(payment): add google_pay_existing_payment_method_required to Manifest at Google Pay

[rdlabo]

Related: https://github.com/capacitor-community/stripe/issues/329

• [x] existingPaymentMethodRequired can change.

If true, Google Pay is considered ready if the customer's Google Pay wallet has existing payment methods.
Default to true.

AndroidManifest.xml:

+        <meta-data
+            android:name="com.getcapacitor.community.stripe.google_pay_existing_payment_method_required"
+            android:value="@bool/google_pay_existing_payment_method_required"/>

string.xml

<bool name="google_pay_existing_payment_method_required">false</bool>