Added Client Certificate Auth blueprint.

[alficles]

This blueprint, if implemented, will allow users and systems to authenticate with TLS certificates in addition to passwords. This will allow implementations considerable flexibility in how they handle passwordless logins.

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Which Traffic Control components are affected by this PR?

• Documentation

What is the best way to verify this PR?

This is a blueprint.

If this is a bugfix, which Traffic Control versions contained the bug?

No bug.

PR submission checklist

• [ ] This PR has tests
• [x] This PR has documentation
• [ ] This PR has a CHANGELOG.md entry
• [x] This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)