Adding CEP for conda_build_config.yaml.

[ryanskeith]

Checklist for submitter

• I am submitting a new CEP: Specification of conda_build_config.yaml variant configuration files.
  • I am using the CEP template by creating a copy cep-0000.md named cep-XXXX.md in the root level.

Checklist for CEP approvals

• The vote period has ended and the vote has passed the necessary quorum and approval thresholds.
• A new CEP number has been minted. Usually, this is ${greatest-number-in-main} + 1.
• The cep-XXXX.md file has been renamed accordingly.
• The # CEP XXXX - header has been edited accordingly.
• The CEP status in the table has been changed to approved.
• The last modification date in the table has been updated accordingly.
• The table in the README has been updated with the new CEP entry.
• The pre-commit checks are passing.