Added support for db migrations

[manjeet-fetchai]

Added support for db migrations:

• Assuming DATABASE_URL, SHADOW_DATABASE_URL, ROOT_DATABASE_URL variables would be created by startup script internally using the available environment variables
  Ref: 787a833

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Code Review Checklist (to be filled out by reviewer)

• Description accurately reflects what changes are being made.
• Either the PR references an issue (via the "Development" combobox) or the description explains the need for the changes.
• The PR appropriately sized.
• The PR contains an idempotent DB migration.
• I have verified the correctness of the DB migration using relevant data (e.g. test-generated data).
• New code has enough tests.
• New code has enough documentation to answer "how do I use it?" and "what does it do?".
• Existing documentation is up-to-date, if impacted.

[bryanchriswhite]

We actually shouldn't need these in production. These are only needed for resetting the DB or committing migrations. We should only ever be applying migrations in production which only interacts with DB_DATABASE.