Refactor `Context` to enforce its invariant statically

[aaronjeline]

Category

Internal refactors/changes

Describe the feature you'd like to request

cedar_policy_core::ast::request::Context looks like:

/// `Context` field of a `Request`
#[derive(Debug, Clone, PartialEq, Serialize)]
pub struct Context {
    /// Context is internally represented as a `PartialValue`
    ///
    /// Context is serialized as a `RestrictedExpr`, for partly historical reasons.
    //
    // INVARIANT(ContextRecord): This must be a `Record`: either
    // `PartialValue::Value(Value::Record)`, or
    // `PartialValue::Residual(Expr::Record)`, or an appropriate unknown
    #[serde(flatten)]
    context: PartialValueSerializedAsExpr,
}

We should enforce this invariant statically if possible

Describe alternatives you've considered

Status quo, enforce it dynamically.

Additional context

No response

Is this something that you'd be interested in working on?

• [ ] 👋 I may be able to implement this feature request
• [ ] ⚠️ This feature might incur a breaking change

[shaobo-he-aws]

I'd like to prioritize this issue as it's been shown to help multiple PRs.

[khieta]

One PR this would have helped is #1027. (Note that the PR also updates the ContextRecord invariant to accurately reflect that a Context can never be purely unknown).

I think the right solution here is to store the context contents as a hash map (or similar data structure) instead of a record expression.