Add extractors for inner variants of MatchedPath and RawPathParams #3880

Feature Request

Motivation

After #3856 we have InnerPath but we're still missing equivalent for MatchedPath and RawPathParams.

Proposal

Add

• InnerMatchedPath
• Expose start of inner match in RawPathParams

Alternatives

Create InnerRawPathParams instead of exposing index of the start. However, that struct is mostly for performance benefits over things like Path<Vec<String>> if you want to avoid deserializations and allocations so it might be preferable to not allocate a new Vec for the new extractor.