fix/759: Add background processing for Embeddings Classification

[Sidsector9]

Description of the Change

This PR implements Action Scheduler to perform Embeddings Classification with 20 terms per job. (The count is opinionated, open to change it).

Closes #759

How to test the Change

1. Visit /wp-admin/tools.php?page=classifai&tab=language_processing
2. Click save.
3. Observe the notice "OpenAI Embeddings: Classification is in progress in the background."
4. Wait until message updates to "OpenAI Embeddings: Classification completed."
5. Verify the embeddings generated in the wp_termmeta table.

Changelog Entry

Added - classifai_openai_embeddings_max_terms filter hook to change the number of terms to process during Embeddings classification per job.
Changed - Embeddings classification now uses background processing to improve performance.

Credits

Props @dkotter @Sidsector9

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[dkotter]

Left a few comments but overall this looks really good and seems to work well.

One other thing we'll need to do before this is ready to merge is bring these same changes over to the Azure OpenAI Embeddings Provider, as right now we only added this to the OpenAI Embeddings Provider

[dkotter]

This looks good and tests well but still need to account for this piece:

bring these same changes over to the Azure OpenAI Embeddings Provider

@Sidsector9 Let me know if you'd like me to tackle that

[Sidsector9]

@dkotter sorry for the delay, I couldn't find time to pick this up. I'll work on Azure and revert soon for review.