Leverage Key module for storage of Vector Storage API details

Problem/Motivation

As spotted in https://www.drupal.org/project/search_api_milvus/issues/3403280 🐛 Clean up API client and Milvus Backend, getting them closer to working state Needs review module by @eleonel the Key module is leveraged there. It makes sense to use the same approach here for Key storage to give developers more flexibility around this.

Steps to reproduce

Attempt to use any Vector Storage client - Keys are stored in config.

Proposed resolution

Add dependency to Key within OpenAI Embeddings sub-module

Remaining tasks

Update code

User interface changes

User must configure Key via Key module, then select it here

API changes

None

Data model changes

Key no longer stored in config here, only machine name of key from Key module.