Improved confirm_localpart

This issue has been migrated from [#14790](https://github.com/matrix-org/synapse/issues/14790).

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**Description:**

For OIDC, you can enable the confirm_localpart option so people can confirm their username.

But sometimes, for instance, you want users to be able to change their display name but not their email address, maybe also not their localpart either.

A drawback of this option is that it is only available for OIDC and not for SAML for instance (unsure if it's there for CAS). So, this feature request is 2 things:

(1) Make what specific user attributes have to be confirmed on signup more configurable
(2) Make this feature available for anyone using SSO, and not just OIDC people

I have never contributed to Synapse before, but I'm willing to try my shot at making a PR for this.

**Edit: Other related improvements**

I do want to send a PR. I plan to submit PR 1 and 2 first since they're most straightforward, then a separate draft PR for 3, and finally 4.

1. Make `confirm_localpart` available to SAML and CAS too (this issue)

2. Make the name and icon customizable for SAML and CAS

3. Make changing/confirming email, localpart and display name independent from each other and each customizable from the homeserver config. (`confirm_localpart` should still be there for backwards compatibility purposes)

4. Let people define custom properties users can confirm or specify when signing up (using a custom template), and pass them to `on_user_registration`.