Add hidden extra sign up fields

[lbalmaceda]

Changes

Previously the extra sign up fields were limited to visible fields of type: name, email, phone number and number. This PR extends the support to hidden fields that are not being displayed in the sign up form but are still sent with a fixed value to the backend as root profile or user_metadata attributes.

I'm not a fan of keeping reference to CustomField type constants from the HiddenField class. Ideally these interface definition should be outside or in the parent abstract class. But doing that now would be a breaking change.

Usage:

Same builder method as before now accepts subclasses of SignUpField class.

Previously

List<CustomField> fields = new ArrayList<>();
fields.add(new CustomField(R.drawable.com_auth0_lock_ic_mobile, CustomField.FieldType.TYPE_NAME, "full_name", R.string.hint_name, CustomField.Storage.PROFILE_ROOT));
builder.withSignUpFields(fields);

Now

Note the method will still work accepting the List<CustomField> argument, but if you want to include HiddenField and any other future field you'd need to change the list type to List<SignUpField>.

List<SignUpField> fields = new ArrayList<>();
fields.add(new CustomField(R.drawable.com_auth0_lock_ic_mobile, CustomField.FieldType.TYPE_NAME, "full_name", R.string.hint_name, CustomField.Storage.PROFILE_ROOT));
fields.add(new HiddenField("reference", "1234", CustomField.Storage.PROFILE_ROOT));
builder.withSignUpFields(fields);

References

• Supersedes Options now pass additional user meta data #549

Testing

• This change adds unit test coverage

• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines

• I have read the Auth0 Code of Conduct

• All existing and new tests complete without errors

• The correct base branch is being used

[lbalmaceda]

Document the supported "keys" when the storage is "root profile". (or link to docs)

[lbalmaceda]

Resolving because this is already added here https://github.com/auth0/Lock.Android/blob/master/lib/src/main/java/com/auth0/android/lock/utils/CustomField.java#L69

[joshcanhelp]

LGTM!