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/changelog.d/17120.bugfix b/changelog.d/17120.bugfix
new file mode 100644
index 0000000000..85b34c2e98
--- /dev/null
+++ b/changelog.d/17120.bugfix
@@ -0,0 +1 @@
+Apply user email & picture during OIDC registration if present & selected.