Add sharding key for `fork_networks`

Sharding keys need to be set for the tables: keys, This involves choosing one of the following, based on the intended behaviour of the table:

• The table is not cell-local
  • Set gitlab_schema to gitlab_main_clusterwide.
• The table is cell-local and requires a sharding key
  • Set gitlab_schema to gitlab_main_cell
  • Add a sharding_key or desired_sharding_key configuration. If the configuration is known but the chosen key doesn't yet meet not-null and foreign key requirements, you can add an exception to allowed_to_be_missing_not_null or allowed_to_be_missing_foreign_key to get the pipeline passing. Please link to a follow-up issue in a code comment next to the exception.
  • You may also need to set allow_cross_joins, allow_cross_transactions and allow_cross_foreign_keys if changing the schema causes pipeline failures. See db/docs/epics.yml for an example.
• The table is cell-local and does not require a sharding key
  • Set gitlab_schema to gitlab_main_cell and
  • Set exempt_from_sharding to true.

Documentation

• Choosing either the gitlab_main_cell or gitlab_main_clusterwide schema
• Defining a sharding key for all cell-local tables
• Defining a desired_sharding_key to automatically backfill a sharding_key