Custom fields for memberships and contributions do not appear on contribution page profiles for anonymous users

Profiles on contribution pages will display custom fields that are applicable to contributions and memberships if the user is authenticated. The fields are not shown for anonymous users.

To replicate:
1. Create a custom field that is applicable to a membership (e.g. Region)
2. Create a profile with some fields for an individual (e.g. First Name) and the custom membership field (Region)
3. Create a contribution page with memberships enabled, and the profile enabled.
4. View the live contribution page as an authenticated user and see all the fields in the profile
5. View the live contribution page as an anonymous user and observe that the custom membership field is not present.

The same pattern can be replicated for custom contribution fields.

Desired behaviour is to have consistent appearance of the profile fields on the contribution pages regardless of authentication status. Either not allowing these fields, or allowing them. I can see some potential usefullness in allowing these custom fields, but note that there is some nuance in populating with default values. For instance if you are renewing an existing membership, then a custom membership field should probably have its default value set to the current custom value associated with the membership. The contribution custom field on the other hand would not have a default value based on any external condition.