urlapi: avoid Curl_dyn_addf() for hex outputs

urlapi: avoid Curl_dyn_addf() for hex outputs

Inspired by the recent fixes to escape.c, we should avoid calling
Curl_dyn_addf() in loops, perhaps in particular when adding something so
simple as %HH codes - for performance reasons. This change makes the
same thing for the URL parser's two URL-encoding loops.

Closes #10384