Define some metadata structure

[aborgna-q]

Currently in the implementation the metadata for a node is just a serde_yaml::Value. This is really prone to cause collisions between (internal and external) users trying to store data there.

The spec doesn't define any structure for it, but it would be beneficial to at least require a HashMap<String, Value>.

Update: #661 Made the metadata always a HashMap, but the string keys do not have any structure yet.

[cqc-alec]

Agreed. Do we need to impose any more structure than HashMap<String, Value>?

[aborgna-q]

I'd say that's enough.

[ss2165]

I suggest rmp::Value rather than YAML, since that will be the most common source.
I agree with HashMap<String, Value> but we may wish to reserve some keys (or reserve a key format), and per reserved key we can impose some schema.

[aborgna-q]

This is currently a serde_json::Map<String, serde_json::Value>.

We could change it to use rmp instead.

[ss2165]

We should probably revisit the format (messagepack/json/whatever) after the wider data models discussion