Add support for unencrypted object metadata

[gyim]

This PR implements the necessary changes for the searchable metadata feature:

1. Added metadata cipher field to access grants. Metadata encryption can be enabled/disabled per folder, just as path encryption.
2. Added clear (unencrypted) metadata field to several protobuf structures. I was thinking about sending the clear metadata in the encrypted_metadata field (just as for paths), but it would cause existing clients to fail without a metadata encryption key. So it was more modification in the common repository, but more backwards compatible.
3. Added a new usermeta package that converts deeply structured JSON from/to serializable metadata. For compatibility reasons, deep metadata structures are supported in the application layer: metadata object {"s":"text","o":[1,2,3]"} are sent as map[string]string in the form of {"s":"text","json:o":"[1,2,3]"}. It is the responsibility of satellite and the client application to encode/decode these shallow JSONs using the usermeta package.