feat(hoppscotch): add existing secret lookup, extra env vars to init containers, global image registry, and custom scheduling for migrations job

[jonathanfoster]

This PR includes the following feature updates:

• feat(hoppscotch): add postgres, redis, and clickhouse existing secret lookup (resolves issue chart does not implement existingSecretSqlConnectionKey #55)
• feat(hoppscotch): add extra env vars to init containers (resolves issue chart does not implement existingSecretSqlConnectionKey #55)
• feat(hoppscotch): add global image registry to hoppscotch container images (resolves issue wait-for-db initContainer does not allow for customizing the image: #56)
• feat(hoppscotch): add custom scheduling to migrations job (resolves issue Migrations need a node selector for some setups #57)
• feat(hoppscotch): increase default resources presets to small to avoid oomkilled status

Unit test coverage was significantly enhanced to fully cover Postgres, Redis, and ClickHouse connection URL handling logic including existing secret lookup.

All changes include unit tests and all end-to-end tests pass.