Update `RouteHandlerFilterContext.Parameters` API

[captainsafia]

Follow-up to #40491

- public sealed class RouteHandlerInvocationContext
+ public class RouteHandlerInvocationContext
{
-  public HttpContext HttpContext { get; }
+ public virtual HttpContext HttpContext { get; }

- public List<object?> Parameters { get; }
+ public virtual List<object?> Parameters { get; }

+ public virtual T GetParameter<T>(int index) { }
}

[captainsafia]

Moving this back into .NET 7 Planning to prioritize completing the performance tests in #40655 for preview3.

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We're moving this issue to the .NET 7 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
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[captainsafia]

We've added performance tests for some basic scenarios with minimal APIs and route handler filters.

• A route handler with no parameters and no filter
• A route handler with no parameters and a "pass through filter"
• A route handler with parameters and a "pass through filter"
• A route handler with parameters and a filter

All the route handlers above tested on a string input/output for the route handler. While testing other input/output types might provide more "real world" values, the bare scenario is sufficient for observing the net impact of the barebones filters logic.

All told, adding a filter to an endpoint reduces throughput (measured in RPS) by about 9%. There's generally a bigger impact when adding parameters to an endpoint filter and parameter processing is a larger cost in the process than endpoint filters.

Regardless of the perf impact, IMO, this work is relevant to pursue because exposing parameters as an untyped list is 🤢 .