Add sharding key for push_rules

What does this MR do and why?

Contributes to #490488

After migrating group and organization specific push rules to separate tables:

• Group push rules: populate `group_push_rules` w... (#498935 - closed)
• Organization push rules: migrate organization p... (#498939 - closed)

push rules table will be responsible for project push rules.

It allow us to set a sharding key per project.

References

Add sharding key for `push_rules` (#490488)

MR acceptance checklist

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Related to #490488