Apply user `email` & `picture` during OIDC registration if present & selected (#17120)

This change will apply the `email` & `picture` provided by OIDC to the
new user account when registering a new user via OIDC. If the user is
directed to the account details form, this change makes sure they have
been selected before applying them, otherwise they are omitted. In
particular, this change ensures the values are carried through when
Synapse has consent configured, and the redirect to the consent form/s
are followed.

I have tested everything manually. Including: 
- with/without consent configured
- allowing/not allowing the use of email/avatar (via
`sso_auth_account_details.html`)
- with/without automatic account detail population (by un/commenting the
`localpart_template` option in synapse config).

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [X] Pull request is based on the develop branch
* [X] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [X] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

1 files changed, 1 insertions, 0 deletions

diff --git a/docs/sso_mapping_providers.md b/docs/sso_mapping_providers.md
index 10c695029f..d6c4e860ae 100644
--- a/docs/sso_mapping_providers.md
+++ b/docs/sso_mapping_providers.md
@@ -98,6 +98,7 @@ A custom mapping provider must specify the following methods:
         either accept this localpart or pick their own username. Otherwise this
         option has no effect. If omitted, defaults to `False`.
       - `display_name`: An optional string, the display name for the user.
+      - `picture`: An optional string, the avatar url for the user.
       - `emails`: A list of strings, the email address(es) to associate with
         this user. If omitted, defaults to an empty list.
 * `async def get_extra_attributes(self, userinfo, token)`