Add sharding key to award_emoji table

This table has a polymorphic association. From a production clone, I see we have 5 different awardables

select distinct(awardable_type) from award_emoji;
 awardable_type
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 Epic
 Issue
 MergeRequest
 Note
 Snippet
(5 rows)

Snippet type might be tricky as it has a mixed sharding key type #460829 (closed)

Sharding key considerations

Important

Marking all tables that were blocked by the notes backfill with this message. Please read through #444222 (comment 2765183660) as it might affect the structure of the sharding key for this table