Add external_id to Contact model

[al3x3i]

Why?

Add external_id because API 2.2 supports it.

How?

Allows have different Contacts with the same email.

[al3x3i]

Fixes: #286

[narenlog]

Hi ! lot of us are waiting to query contact based on external_id which is the userID from the User object which is not available anymore.

In any case, external_id is the unique identifier when we have multiple contacts with the same email.
You can see from the forum posts how important it is to query external_id

Please review and merge please.

• https://www.intercom.com/help/en/articles/268-what-is-user_id-and-why-would-i-want-to-use-it
• https://forum.intercom.com/s/question/0D52G00004xA07XSAS/is-the-user-id-the-same-as-the-external-id-on-the-contact-api
• https://forum.intercom.com/s/question/0D55c00005ogPvNCAU/is-it-possible-to-find-exactly-one-contact-by-their-external-id-or-call-delete-on-exactly-one-contact-by-their-external-id
• https://forum.intercom.com/s/question/0D52G00004q6eCUSAY/search-for-contacts-using-the-in-operator