Support unsigned id_tokens for OIDC providers

This issue has been migrated from [#9250](https://github.com/matrix-org/synapse/issues/9250).

---

### Description

I enable OpenID Connect in Synapse to login with user accounts located in an OpenID Connect provider.

I'm testing my server configuration using the client on https://app.element.io.

The following happen:

* I configure my custom server
* I hit `Sign in with single sign-on` button
* I'm redirected to my OIDC provider where I login
* I'm then redirected `https://matrix.atelier-medias.org/_synapse/oidc/callback?code=************` whith the following error displayed:

```
There was an error during authentication:
unsupported_algorithm:

If you are seeing this page after clicking a link sent to you via email, make sure you only click the confirmation link once, and that you open the validation link in the same client you're logging in from.

Try logging in again from your Matrix client and if the problem persists please contact the server's administrator.

Error: invalid_token
```

in the following in the log:

```
2021-01-28 14:10:36,898 - synapse.http.client - 429 - INFO - GET-7 - Received response to POST https://intra.atelier-medias.org:443/xwiki/oidc/token: 200
2021-01-28 14:10:36,899 - synapse.handlers.oidc_handler - 671 - ERROR - GET-7 - Invalid id_token
Traceback (most recent call last):
File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/synapse/handlers/oidc_handler.py", line 669, in handle_oidc_callback
userinfo = await self._parse_id_token(token, nonce=nonce)
File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/synapse/handlers/oidc_handler.py", line 459, in _parse_id_token
claims_params=claims_params,
File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/authlib/jose/rfc7519/jwt.py", line 99, in decode
data = self._jws.deserialize_compact(s, load_key, decode_payload)
File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/authlib/jose/rfc7515/jws.py", line 102, in deserialize_compact
algorithm, key = self._prepare_algorithm_key(jws_header, payload, key)
File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/authlib/jose/rfc7515/jws.py", line 249, in _prepare_algorithm_key
raise UnsupportedAlgorithmError()
authlib.jose.errors.UnsupportedAlgorithmError: unsupported_algorithm:
```

I'm not fully sure what it exactly means by "unsupported_algorithm" but this OpenID Connect provider returns plain tokens (not encrypted or signed tokens) which is perfectly valid from OpenID Connect protocol point of view, but maybe it's related ? Just in case, I tried to set `skip_verification: true` but it does not seems to help.

### Version information

Synapse 1.26 installed with matrix-synapse-py3 on Debian Stretch.
Python 3.5 (I noticed a warning about the Python version but that's the version I got on Debian Stretch, so I hope it's not related...)