Support Apollo federation @key for unique key columns

### Describe the solution you'd like

When I enable Apollo Federation on a table I would like a `@key` for the primary key but also would like `@key` for each unique column key as well. `@key` is a repeatable directive so a entity/table may have multiple keys

No files, tests, or entry points are identified in the issue, so first locate the Apollo Federation implementation and its handling of table primary and unique columns. Done means an entity can expose an Apollo Federation @key for its primary key and for each unique column key, with multiple repeatable @key directives supported.

Written by the indexing model from the issue text.