Support for user-defined contexts

[vazirim]

Is your feature request related to a problem? Please describe.
In PDL, we only have one background context pdl_context which is handled mostly implicitly.

This proposal will add user-defined contexts as first-class entities.
contribute will be used to indicate what contexts a block is contributing to (including None).
Nested blocks will inherit this set of contexts.

Describe the solution you'd like

Describe alternatives you've considered

Additional context

[esnible]

Can you give a motivating example? Perhaps something simple like asking one model to write code and another model to analyze it?