Adds container_registry_size to project_statistics

What does this MR do and why?

So that we can track and eventually display Container Registry storage size, this MR adds a new container_registry_size column to the ProjectStatistics table.

With this column we can eventually (in upcoming MRs) update the column when Container Registry pushes/deletes happen

Refs #354480 (closed)

Migration info

Migrate up:

> bin/rails db:migrate

== 20220404114106 AddContainerRegistrySizeToProjectStatistics: migrating ======
-- add_column(:project_statistics, :container_registry_size, :bigint, {:default=>0, :null=>false})
   -> 0.0111s
== 20220404114106 AddContainerRegistrySizeToProjectStatistics: migrated (0.0111s)

Migrate down:

> bin/rails db:rollback

== 20220404114106 AddContainerRegistrySizeToProjectStatistics: reverting ======
-- remove_column(:project_statistics, :container_registry_size, :bigint, {:default=>0, :null=>false})
   -> 0.0132s
== 20220404114106 AddContainerRegistrySizeToProjectStatistics: reverted (0.0166s)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.