converting flutter-pluggable-ci-design into rfc - #29

converting flutter.dev/go/flutter-pluggable-ci-design into rfc

Pre-launch Checklist

• I read RFC 000.0001: Taxonomy and followed the file and path naming and metadata standards.
• I read RFC 000.0002: Process and confirmed this proposal meets the threshold for a full RFC.
• I read and agree to the Code of Conduct.
• I signed the CLA.
• I have linked an issue from flutter/flutter with the label design doc.
• All existing and new tests are passing.
• I have enabled "Allow edits from maintainers" on this PR so the bot can automatically assign an RFC number (or I will run dart run bin/assign_rfc_number.dart locally when instructed).

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