Add organization_id column to runner tables

As decided in #470872 (comment 2382046293), we'll replace sharding_key_id by organization_id. The first step will be to add the column to the following tables: ci_runners, ci_runner_machines, and ci_runner_taggings (both non-partitioned and partitioned versions).

Follow instructions in documentation.

• Release N.M:

  • Column will be added as nullable to account for instance runners. We'll need to backfill group and project runners later on.
  • Add index for organization_id: index_ci_runners_on_organization_id_when_not_null" btree (organization_id) WHERE organization_id IS NOT NULL
  • Add LFK to organizations table
  • Add logic to populate organization_id in models.

• Release N.M+1:

  • Add logic to validate organization_id in models.
  • Add PDM with check constraint for organization_id nullness depending on runner_type (similar to check_sharding_key_id_nullness)

  Examples: Ci::Runner#ensure_manager, Ci::Runner sharding_key_id validations, Ci::RunnerManager sharding_key_id validations, Ci::RunnerTagging sharding_key_id validation, Ci::Runners::RegisterRunnerService#attrs_from_token.