Disambiguate payment acct json form's "country" field

Make it clear the user needs to enter a two letter country code
in json form's "country" field, not a country name.

The json form's field name was not changed to "countryCode" in
this change for the following reason:

The API's payment account forms are dynamically generated using
reflection, and PaymentAccount class hierarchy @Setter methods
determine which form fields are included or excluded.  Reflection
and @Setter methods are also used when reading completed json
forms, to set the field values on a new PaymentAccount instance
before it is persisted.

CountryBasedPaymentAccount subclasses have a country field and a
@Setter, not a countryCode field, hence this shortcut to informing
the user the country field value is a country-code.

apitest/src/test/java/bisq/apitest/method/payment/AbstractPaymentAccountTest.java

@@ -129,7 +129,10 @@ protected final void verifyEmptyForm(File jsonForm, String paymentMethodId, Stri
         assertEquals(paymentMethodId, emptyForm.get(PROPERTY_NAME_PAYMENT_METHOD_ID));
         assertEquals("your accountname", emptyForm.get(PROPERTY_NAME_ACCOUNT_NAME));
         for (String field : fields) {
-            assertEquals("your " + field.toLowerCase(), emptyForm.get(field));
+            if (field.equals("country"))
+                assertEquals("your two letter country code", emptyForm.get(field));
+            else
+                assertEquals("your " + field.toLowerCase(), emptyForm.get(field));
         }
     }
 

core/src/main/java/bisq/core/api/model/PaymentAccountTypeAdapter.java

@@ -147,7 +147,10 @@ private void writeInnerMutableFields(JsonWriter out, PaymentAccount account) {
 
                     String fieldName = field.getName();
                     out.name(fieldName);
-                    out.value("your " + fieldName.toLowerCase());
+                    if (fieldName.equals("country"))
+                        out.value("your two letter country code");
+                    else
+                        out.value("your " + fieldName.toLowerCase());
                 }
             } catch (Exception ex) {
                 String errMsg = format("cannot create a new %s json form",