feature(engine): allow customizing execution parameters per context

[rgrinberg]

Contexts are a way for users to customize the rules in a workspace and build all these workspaces concurrently. This PR makes it possible to customize the execution parameters per context, giving us additional customization possibilities for free.

In addition, it's also good to make the engine as agnostic to contexts as possible. It gives the rules more flexibility and makes the engine assume less about the rules.

Here's a related ticket that describes the motivation more: #7049

[rgrinberg]

@pmwhite could you review this one as well?

[pmwhite]

Looks good. I don't see where the behavior changed, though. Would it be easy enough to write a test that demonstrates using different execution parameters in different contexts? Or if that's too much of a pain, maybe just elaborate in this PR a bit more. What you said seems clear enough to someone with more context, but I don't have very much - at least not about this corner of Dune's feature set.

[rgrinberg]

I don't see where the behavior changed, though

There should be no change in behavior.

Would it be easy enough to write a test that demonstrates using different execution parameters in different contexts?

It should be easy, but there's no way to do this yet. We can only modify the execution context via project files and there's no way to do that yet.

I'll update the description to include more context.

[pmwhite]

Ahh, I see now. We're expanding what the engine can do, but not changing behavior for existing use-cases. Makes sense! Thanks for explaining.