fix: Corrected lack of optional parameters for creating multiple mailing list members

[leovietro]

Hello,

I encountered an issue while integrating this SDK into our project. Our mailing lists only store email addresses and ignore the other optional parameters defined in the CreateUpdateMailListMembers type:

type CreateUpdateMailListMembers = {
    address: string;
    name?: string;
    vars?: string;
    subscribed?: 'yes' | 'no' | boolean;
    upsert?: 'yes' | 'no';
}

However, the IMailListsMembers.createMembers method requires the members parameter to use the MailListMember type, which mandates all properties:

type MailListMember = {
    address: string;
    name: string;
    subscribed: boolean,
    vars: {
        [key: string]: unknown
    };
}

Our temporary workaround was to cast the parameter to any, but we don't like that :P

return await client.lists.members.createMembers(listAddress, {
      members: members.map(
        (address) =>
          ({
            address,
            // This cast was made because of an error on the Mailgun SDK typing that makes some MailListMember optional properties obligatory.
          }) as any,
      ),
      upsert: 'yes',
    });

To resolve this cleanly, I created a new type for IMailListsMembers.createMembers that makes these properties optional. Although I considered using the existing CreateUpdateMailListMembers with Omit<> to remove the upsert field, a new type looks clearer and simpler for casting:

export type CreateMultipleMailListMembers = Array<{
    address: string;
    name?: string;
    vars?: string;
    subscribed?: boolean;
}>