Extract list of flow providers to a field
Extract list of flow providers to a field

This is the first step toward being able to add more providers on a
per-TLD basis.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122656117

google/nomulus,google/nomulus,google/nomulus,google/nomulus,google/nomulus,google/nomulus