Add AbstractKVStoreIntegrationTest and PostgresImpl Test

[G8XSU]

Based on #5

How?

• PostgresIntegrationTest uses testcontainers to spin up docker containers for each test.
• AbstractKVStoreIntegrationTest defines behavior that every impl of KVStore needs to follow. Different impl's can re-use the same abstract class to test their impl against it.
• We will keep on adding tests to this as we add more impl details.

Files to review:

• AbstractKVStoreIntegrationTest
• PostgresBackendImplIntegrationTest

[G8XSU]

Reviewed and merged as part of another PR