add agenda item for unions implementing interfaces (#919)

* add agenda item for unions implementing interfaces

   - Spec PR: graphql/graphql-spec#939
   - graphql-js PR (WIP): graphql/graphql-js#3527

* Update agendas/2022/2022-04-07.md

Co-authored-by: Benjie Gillam <benjie@jemjie.com>

agendas/2022/2022-04-07.md

@@ -120,3 +120,6 @@ This is an open meeting in which anyone in the GraphQL community may attend.
 1. Discuss asynchronous iterators of iterators versus of items (15m, Yaacov)
    - Is spec update required for this?
    - Background: https://github.com/robrichard/defer-stream-wg/discussions/38
+1. Discuss proposal to allow unions to declare implementation of interfaces (15m, Yaacov)
+   - Spec PR: https://github.com/graphql/graphql-spec/pull/939
+   - graphql-js PR (WIP): https://github.com/graphql/graphql-js/pull/3527