Add metadata to each node

[chrislearn]

There is a lack of structured information description of the node.

We can add Dora.toml/Node.toml files in each Node to provide some basic descriptions of it.

This helps users understand how to use it without looking at the internal code of the node.

Other tools or programs can also use these metadata to determine whether the configuration of dataflow.yml is valid.

[LeonRust]

This feature needs to be designed together with the package management system.