Store E2EE Keys without an additional password

This issue has been migrated from [#12386](https://github.com/matrix-org/synapse/issues/12386).

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E2EE seems to be the way forward. Which i consider to be a good thing.

But not all users (or all use-cases) are paranoid about E2EE. There are some issues open to disable E2EE enforcement or at least nudging on Homeserves (e.g. https://github.com/matrix-org/synapse/issues/4367, https://github.com/matrix-org/synapse/issues/4551 https://github.com/matrix-org/synapse/issues/4401).

Matrix seems to get a lot of traction in Corporate Environments (or similar environments like universities, schools, kindergarten) where e2ee is considered to be important but usability is considerable a key factor as well. Having the possibility to use e2ee and not disable it and still be able to participate in a federated environment (even with a warning) could be beneficial to such considerations and generally help the push towards e2e.

An solution would be to enable the possibility to use the Key Storage without an additional password. Just store it. Maybe even warning the user user, that this might pose an security risk to store it without encryption. It might even be considerable to differentiate between "highly secure" and "secure" conversations according to the way users have chosen to store their e2ee keys.