Add get_string convenience method to Context for string value extraction

Describe the improvement you'd like to request

Problem

When working with Context objects, extracting values requires verbose pattern matching through the internal AST structure. This creates unnecessary boilerplate code and complexity for a common operation.

Proposed Solution

Add a get convenience method to Context that would simplify value extraction:

impl Context {
    /// Retrieves a value from the Context by its key.
    pub fn get(&self, key: &str) -> Option<&Value>
}

// Current approach requires complex pattern matching
// New approach would be:
let ray_id = request
            .context()
            .and_then(|ctx| ctx.get("rayId"))
            .map(|id| id.to_string())
            .unwrap_or_else(|| "default_id".to_string());

Let me know if you'd like any adjustments to the issue title or description!

Open PR: https://github.com/cedar-policy/cedar/pull/1318

Describe alternatives you've considered

No response

Additional context