curl-rustls.m4: add pkg-config support to rustls detection

curl-rustls.m4: add pkg-config support to rustls detection

Based on the existing openssl pkg-config detection, this commit tries to
use pkg-config to find `rustls` then falls back to the current approach
if that fails.

We use the following logic:

- if no path is provided, just use pkg-config, if it's not there we have
  a problem!
- if a path is provided, try pkg-config
  + if pkg-config fails, try and find rustls directly

Closes #13179