Support dataSource context on regular dataSources

[azf20]

A dataSource can have context, but this is not supported as part of the subgraph.yaml. Users should be able to add context to their manifest which can be accessed in the runtime

[leoyvens]

Sounds fun, we could allow a free-form yaml map which is converted into a context.

[julien-devatom]

is this feature on the roadmap?