config: fix file-backed http_headers path rebasing

[immanuwell]

@roidelapluie @gotjosh small fix here, easy miss.

Headers.SetDirectory mutates Header values from a map copy, so paths from http_headers.files never get written back.
That makes file-backed headers fail when the HTTP config is loaded from a nested path, kind of a footgun.

This just writes the updated header back into the map and adds coverage for the helper and the real LoadHTTPConfigFile plus NewClientFromConfig path.

Repro on main:

1. Put an HTTP config in configs/http.yml.
2. Set http_headers to files: [configs/header-value].
3. Load it with LoadHTTPConfigFile, build a client, send a request.
4. The request fails with unable to read headers file.

With this patch it reads the file and sends the header.