Use a new table/model for Prometheus cluster integration

What does this MR do?

For ease of migration later, use a new table/model for Prometheus cluster integration (https://docs.gitlab.com/ee/user/clusters/integrations.html#prometheus-cluster-integration).

!55244 (merged) introduced this feature in the same table but I regret this. It will be much easier to have a whole new model, so that we can perform the migration from old table to new table at our leisure.

Related issue: #292460 (closed)

Screenshots (strongly suggested)

Screen_Recording_2021-04-14_at_12.58.54_PM

Screen_Recording_2021-04-14_at_1.01.48_PM

Does this MR meet the acceptance criteria?

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