PADV 1684 - Add feature flag for LLM summarize for frontend

[alexjmpb]

Description:

This PR is part of the AI assistance project and aims to add the connection to the summarize backend to the annotator js llm_summarize plugin. The plugin sends a request to pearson core summarize api and replaces the annotation text area value with the summary.

How to test:

• Setup devstack and pull changes.
• Setup edx notes api service.
• Setup Pearson core plugin
• Follow instructions to enable LLM summarize endpoint in pearson core.
• Enable the waffle switch 'pearson_core.llm_summarize.enable_llm_summarize'
• On studio, select a course, go to advance settings, and on the 'other course settings field' put the following JSON:

{
  "llm_summarize": {
    "is_enabled": true
  }
}

• Select a text inside a text or HTML block.
• Click on the summarize button.
• A loader should appear indicating that the content is being summarized.
• Now disable the switch or the other course settings, either way, the summarize button should not appear now.

Reviewers:

• @Serafin-dev
• @Squirrel18