Attributes refactor and specification

[gavento]

The attributes are currently not well-specified and there seem to be ways to implement them better before we release the (much more stable) v0.3.

Goals:

• Rust structures (with serde) with typed values for known attributes (and a map for user/unknown)
• Split Task and DataObject attributes (required by the above, will allow checking in Python and other clients)
• Specify a fixed encoding for rich and user-specified values (current proposal: JSON with base64 binary data) (note that non-trivial binary data in attributes are generally discouraged)
• Create a rich Attributes class in Python (underway in WIP Classes for tasks and Attribute refactor #60)
• Fix and document the new structure semantics

[spirali]

Schema for spec/info transfers

[gavento]

Resolved in v0.3 both in client, server and executor, documentation updated to describe individual attributes.