Revamp 'api_handler()'

[fregataa]

Motivation  

In version 25.02, we introduced the ai.backend.common.api_handlers module to standardize HTTP API handler functions. However, the module has several issues that need to be addressed:

• The name of a generic type variable T does not follow Python naming conventions
• The _parse_and_execute_handler() function only accepts ai.backend.common.APIResponse objects
• The module does not support aiohttp.web.StreamResponse as a return type for API handlers, despite aiohttp.web.Response being a subclass of StreamResponse
• The type checking implementation for API handler responses is incorrect
• API handler signatures are validated at runtime when handling requests, rather than during static analysis or server initialization
• Even when the request is valid, an 'Invalid parameter' error may be raised if the API handler is incorrectly defined.