Improve support for pending members in Core API

Our membership sales team often gets requests from their clients to remove a list of email addresses. This is usually a cleanup operation after staff leaves a given organization.

I have written a script leveraging the Core API to remove active members by looking the customers up by email address and then filtering members by user_id. This works great and the sales team is happy with the results.

The Core API currently gives me no way to locate pending memberships given an email address. Removing these currently requires exporting a list of pending members using the platform UI, and then matching that list to the email list provided by the client.

Please make the following changes to the Core API:

• Add the invitation email address to the response body for member-related requests.

• Extend the filter parameters for the “list members” operation to allow filtering by invitation email address.