Define a sharding key for snippet_repositories

Table snippet_repositories should meet the guidelines for sharding so it can be linked to an Organization: https://docs.gitlab.com/development/organization/#defining-a-sharding-key-for-all-organizational-tables

When the work is complete, the new column should fulfil the following criteria:

• has no default
• is not nullable
• has a foreign key to organizations

Deliverables

[UPDATE]

It turns out this case is more complex than the original steps we have outlined here. I have added the remaining steps below. I will create new issues for each of them to link to.

I have copied these steps from @olaoluro's similar work here: #522958 (closed) as we have established it is an acceptable outline for this work (see comment on this issue: #550705 (comment 2758040195)). I have added a few extra steps that were discovered along the way.

• Add organization_id to snippet_repositories, NULL, has no default, has a foreign key to organizations:
• Add support in code for new column organization_id on snippet_repositories:
• Backfill snippet_repositories records with organization_id:
• Finalize BBM above
• Add validations in code for new column organization_id on snippet_repositories after backfill:
• Add the NOT NULL constraint (after backfill migration is done) to move closer to sharding goal
• Handle shard_id FK (multi step process)
  • Remove NOT NULL constraint from shard_id column. Issue: Remove NOT NULL constraint from snippet_reposit... (#577964 - closed)
  • Update LFK script to allow us to run LFK script on some ON DELETE RESTRICT columns. Issue: Replace `shard_id` FK with LFK for snippet_repo... (!209036 - merged)
  • Run LFK script on snippet_repositories.shard_id. This is being handled in the same MR/update as the script update: Replace `shard_id` FK with LFK for snippet_repo... (!209036 - merged)
• Add organization_id sharding key (this issue)