Finishing TODO in pingora-header-serde for checking if path is a file + avoiding panics

In
https://github.com/cloudflare/pingora/blob/e6e677fe9b58555140ab7bd14feff035392b3530/pingora-header-serde/src/dict.rs#L23-L30

The TODO can be easily completed and the return type can be changed to `Result, Error>` to account for invalid paths (and avoid panics on unwrap).

I will be making a PR soon after this issue is published - I made the minimum required changes and all the tests in the same [file](https://github.com/cloudflare/pingora/blob/main/pingora-header-serde/src/dict.rs) pass with `cargo test` from the subdirectory and `cargo test -p pingora-header-serde` from root.

Thanks!

Om

Start at pingora-header-serde/src/dict.rs lines 23-30 and read the TODO and nearby tests. Run cargo test from that subdirectory and cargo test -p pingora-header-serde from the repository root; done means invalid paths are handled through the Result return type without panics and the existing tests pass.

Written by the indexing model from the issue text.