refactor(framework) Remove partition_id from Context

[jafermarq]

This PR:

• Removes the partition_id from Context (this information can be embedded in the <Context>.node_confg)
• Because of the above the NodeState class also removes the partition_id from its constructor.
• In simulation, the node_config dict carries both partition-id and num-partitions keys
• The above keys are now captured in flwr.common.constants to ease any further changes to these.
• Some simulation tests were creating a Context obj directly. Now they instead create a NodeState object and use it to register & retrieve the context. No shortcuts.