add getOpenIdToken to embedded client backend

[toger5]

Fixes: #3538
This adds oidc token support to the embedded backend.

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

✨ Features

• add getOpenIdToken to embedded client backend (#3571). Fixes Add openId widget api event as defined in MSC1960 #3538. Contributed by @toger5.