box: let authentication method require encryption

If an IO stream is encrypted, it should set flag IOSTREAM_IS_ENCRYPTED.
If an authentication method requires the channel to be encrypted, it
should set flag AUTH_METHOD_REQUIRES_ENCRYPTION. An attempt to use an
authentication method that requires encryption over an unencrypted IO
stream will raise an error ER_UNSUPPORTED("Authentication method '%s'
does not support unencrypted connection"). This check is performed by
both net.box and applier.

Needed for https://github.com/tarantool/tarantool-ee/issues/322

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