Allow SAML/SCIM providers to provide a profile picture URL when provisioning users

### Problem Statement

Right now, the SAML2 and/or SCIM authentication methods allow you to customize the subject user, however, they do not allow you to customize their profile picture. If you are using an IdP, this can be a bummer because not everyone is keen on changing their profile pictures on every platform (Thus sort of defeating the point of an IdP).

### Solution Brainstorm

Add a way for SAML2 and/or SCIM to pass the profile picture (Through a link) to Sentry, for that profile picture to then be the provisioned user's Sentry profile picture. Whether they can change it after authentication or not is up to you and I don't think many people will care about this.

### Product Area

Settings - General

Start by tracing the SAML2 and SCIM user-provisioning flows and how they currently customize provisioned users. Determine how an IdP-provided profile picture URL should be carried into the Sentry profile, then verify that provisioning applies the URL and that the resulting profile behavior matches the chosen policy.

Written by the indexing model from the issue text.