fix: adding SQL migration for making device_token UNIQUE

[geekbrother]

Description

This PR is a follow-up for #196 with the following SQL migration changes wrapped into the transaction by adding the new 1695024779_add-unique-device-tokens migration file:

1. Clear the clients table with duplicate device tokens keeping the latest one,
2. Adding UNIQUE constraint to the device_token column in the client's table.

Resolves #201

How Has This Been Tested?

Apply or cherry-pick changes from #211 and #213.
When running the functional tests migrations are applied that's why we can rely on our upsert functional tests from #196 to test that:

• SQL migration in the new file is correct and runs successfully,
• Changes from the SQL migration don't break the current logic.

To run functional tests start the PostgreSQL instances:

docker run \
  -p 5432:5432 \
  -e POSTGRES_HOST_AUTH_METHOD=trust \
  --name echo-server-pg \
  -d postgres

docker run \
  -p 5433:5432 \
  -e POSTGRES_HOST_AUTH_METHOD=trust \
  --name echo-server-tenant-pg \
  -d postgres

Call tests:

cargo t --features functional_tests

The expected behavior tests should pass.

Merging policy and dependency

This PR depends on and should be merged after #213

Due Diligence

• Breaking change
• Requires a documentation update
• Requires a e2e/integration test update

[geekbrother]

Updated the migration filename (increased timestamp) to make it applied after #211