Introduce Ballerina OpenAI Assistants connector

[NipunaRanasinghe]

Description:
To align with the new Ballerina connector reviving effort, we need to introduce a new Ballerina connector for OpenAI Assistants API, to support their latest REST API version in Ballerina.

Following high-level tasks should be completed with this effort:

• Create a new repository + setup the project structure and workflows
• Sanitize the OpenAPI spec (if needed) and, generate the client connector
• Implement a test suite for the connector
• Improve the documentation with setup guide and quickstart guide
• Add example usecase for the connector

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[NipunaRanasinghe]

As of now all the connector action items are complete, but the release is blocked due to the compiler issue: ballerina-platform/ballerina-lang#40606