Readme: Add OpenSearch MCP server to Official Integrations

[rithin-pullela-aws]

Description

Links OpenSearch's official MCP server from readme. Find more about OpenSearch's MCP server here

Motivation and Context

This is OpenSearch's official MCP server, so adding it here.

How Has This Been Tested?

Tested with Claude desktop and langchain.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation update

Checklist

• [x] I have read the MCP Protocol Documentation
• [x] My changes follows MCP security best practices
• [x] I have updated the server's README accordingly
• [x] I have tested this with an LLM client
• [x] My code follows the repository's style guidelines
• [x] New and existing tests pass locally
• [x] I have added appropriate error handling
• [x] I have documented all environment variables and configuration options

Additional context