Fix atscfg Service Category Header to be Internal

[rob05c]

Fixes the TC Service Category header to be internal to ATS, and not sent by ATS in HTTP requests or responses.

Docs already exist for Service Category.
No changelog, no interface change.
Tested by existing Service Category Header Rewrite tests. I also manually tested, verified the @ header is not sent by ATS, and verified the header rewrite is created for both Edges and Mids.

• This PR fixes #REPLACE_ME OR is not related to any Issue

Which Traffic Control components are affected by this PR?

• Traffic Ops ORT

What is the best way to verify this PR?

Run unit tests.
Generate config, verify header rewrites are prefixed with @ to make them internal to ATS. Run ATS, verify header writes are not passed upstream or downstream and are logged if an appropriate svc=%<{@CDN_SVC}cqh> log field exists.

If this is a bug fix, what versions of Traffic Control are affected?

master

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[alficles]

This looks good. It's a pretty simple change. I would note that this is a security improvement when origins are configured to respond to TRACE requests. (They generally shouldn't be.) Previously, these categories were leaked publically in that case. For most use cases, this probably isn't a huge risk, but this is an improvement in any case.

[dsouza93]

I tested this as well for what its worth. It looks good to me