VDB provider should specify dependencies

Problem/Motivation

It's possible for there to be config sync issues where, as part of the sync, the Search API server configuration gets synchronized before ai_vdb_provider_postgres.settings. In this instance, ai_search will attempt to ensure the configured collection exists in the server but fail in the process because it doesn't have the connection information from ai_vdb_provider_postgres.settings, killing the sync process.

Steps to reproduce

Proposed resolution

Implement DependentPluginInterface as the work done in https://www.drupal.org/project/ai/issues/3543030 will bring in VDB plugin dependencies.

Remaining tasks

User interface changes

API changes

Data model changes