feat: add opensearch provider

[n0ta26]

Closes #740

📑 Description

This draft PR introduces an OSS OpenSearch provider to Keep and validates it with a basic workflow against a local OpenSearch container.

Changes

• Implemented a new opensearch provider to integrate OSS OpenSearch with Keep.
• Added and started an OpenSearch container locally to validate the integration.
• Verified that the example workflow using the opensearch provider can successfully run end-to-end (query + index/document operations).

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

ℹ Additional Information

If the implementation looks good direction-wise, I would like to follow up with documentation updates.

Memo

• Added documentation for the OpenSearch provider and implemented a full index search example.
• Planning to add support for receiving alerts from OpenSearch in a future change.
• Planning to add branching logic for index creation and related operations in a future change (for now, the design is aligned with the OpenSearch Serverless provider).

[vercel]

@n0ta26 is attempting to deploy a commit to the KeepHQ Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

All committers have signed the CLA.

[n0ta26]

I’ve updated the PR title.

[greptile-apps]

Skipped: This PR does not target one of your configured branches: (refactor/2512-nextjs-15)