Add config variable table

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What does this MR do?

Create table for CI/CD Configuration Variables.

• currently three values (per !83972 (merged)): CI_SERVER_TLS_CA_FILE, CI_SERVER_TLS_CERT_FILE, and CI_SERVER_TLS_KEY_FILE

Since predefined variables don't have conditions (and other differences), I thought it made most sense to separate the configuration variables into a new table. I took the above three config variables conditions based on reading the Related Issues and these pages:

• https://gitlab.com/gitlab-org/gitlab-runner/-/blob/428076fbc1efe26da7efcf6e489a30c60ca67d2d/common/config.go#L764-766
• https://docs.gitlab.com/runner/configuration/advanced-configuration/#the-runners-section

Let me know whether this is inaccurate. I'm happy to add more config variables too.

Related issues

#364359 !83972 (merged) !190928 (merged)

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