Provide a way to use a shared secret authenticator for double puppet

[MayeulC]

First mentioned here and there

The idea is to automatically allow the bridge to log-in as the user.

This requires adding a new authenticator to Synapse: https://github.com/devture/matrix-synapse-shared-secret-auth

Howto: https://docs.mau.fi/bridges/general/double-puppeting.html#shared-secret-method

1. Install the python file in the synapse python path
2. Add the module to the config
3. Generate a secret value
4. Put that secret value under the signal bridge config as a map:

   bridge:
     login_shared_secret_map:
         my_server.tld: aaaaaaaaasecretaaaaa

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Q: should this be provided as a separate app? The same authentication module can be used by multiple appservices/bridges.

[Thatoo]

I guess the best way would be first to contribute in here : YunoHost-Apps/synapse_ynh#298