Add soft logout possibility for OIDC backchannel_logout

This issue has been migrated from [#15976](https://github.com/matrix-org/synapse/pull/15976).

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### Pull Request Checklist

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog file](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#changelog).
* [x] Pull request includes a [sign off](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#sign-off)
* [x] [Code style](https://matrix-org.github.io/synapse/latest/code_style.html) is correct

- ISSUE : When doing a backchannel logout using an OIDC provider, a user may lose any non backed-up keys.

- PROPOSED FEATURE : Allow to set the backchannel logout induced by an OIDC to be a soft-logout, thus allowing the user to reconnect to the same device and recover his session if necessary.

- IMPLEMENTATION : Allow setting OIDC backchannel logout to be a soft-logout in OIDC provider configuration via `backchannel_logout_is_soft` which defaults to false.

Signed-off-by: OUERTANI Mohamed Hachem