extract LoadPrivateKeyFromFile from embedded_test_server to cert_test_util

Bug: 1266634
Change-Id: I6cf72235a37954f365f6dffe3f9a017f0e2c8706
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3378698


Reviewed-by: Hubert Chao <hchao@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#958195}