Openapi schema update (#20)

This pull request includes substantial changes to the `openapi.yaml`
file, primarily focusing on restructuring the API endpoints and
enhancing the schema definitions. The most important changes include
updating the API paths, adding detailed contact and license information,
and refining the schema definitions for better clarity and usability.

### API Path Updates:
* Changed the endpoint for listing chat histories from `/llm-providers`
to `/api/v1/chats` and updated the operationId and summary accordingly.
* Added a new endpoint `/api/v1/chats/{chatId}` for retrieving a single
chat by UUID, including necessary parameters and responses.
* Introduced a new endpoint `/api/v1/chats/stream` for streaming chat
conversations with detailed request and response structures.
* Updated the endpoint for listing tools from `/api/tools` to
`/api/v1/tools`.
* Reintroduced the endpoint `/api/v1/llm-providers` for getting a list
of supported LLM providers and models.

### Schema Enhancements:
* Added examples and refined descriptions for properties in various
schemas such as `ModelSettings`, `LLMProvider`, `SupportedLLMProviders`,
`Message`, `ChatHistory`, `QuestionRequest`, `StreamChatRequest`, and
`Response`.
* Introduced a new `StreamSettings` schema to define settings for
streaming responses.
* Improved the `Message` schema to include a `Role` property and updated
the `generated_at` description.

### Additional Information:
* Added contact and license information to the API documentation,
including the name, email, and license URL.
* Defined a default local HTTP server in the `servers` section.

Author: shahariaazam