lakefs kv cmd

[itaidavid]

closes #4121

lakefs kv get and lakefs kv scan implemented. There is a generic value -> proto -> json converter, that allows each module (auth, graveler, etc.) to associate different types with different path format, and so each value is formatted correctly according to the path is is brought from. This will also support future addtion of types without the need to change this tool It still falls short with staged objects, as their value is also a proto message which needs another translation

Only tested manually at this point