Meta Description Generator: Generate meta descriptions via AI connector

### Description

>[!TIP]
>[Meta Description Generator feature reference document for Alley devs](https://l.alley.dev/0e3715e20a)

Set up the core functionality to generate meta descriptions using the configured AI connector (see #162 and #163).

The adapter will utilize the configured AI Connector to query the LLM with a filterable prompt and the data for a post (title and rendered post content). This logic will be used in the post admin component (#165) and the CLI functionality (#166).

To make this tool useful for our clients, the prompt needs to be agnostic, not "salesy". The prompt should tell the LLM exactly what to return for easier processing (e.g. "the response will be used in code, please give three options and add line breaks between them").

Be sure to handle bad data properly (empty response, bad status code, timeout, over quota, etc.). Return the warning or error encountered so that it can be handled properly in the admin or CLI.

Ensure this functionality is available via the REST API for people who have edit post permissions so that it can be incorporated into the block editor in #165

### Use Case

When an editor or developer utilizes the post admin component or CLI functionality, the configured AI connector should generate and return meta descriptions.