feat: allow OpenAPI spec overrides

[devin-ai-integration]

Implements user-level OpenAPI overrides in the @api decorator.

This PR adds support for customizing OpenAPI specification fields in BentoML services by:

1. Adding openapi_overrides parameter to @api decorator
2. Adding openapi_overrides field to APIMethod class
3. Implementing merge logic to combine user overrides with auto-generated spec

Example usage:

@bentoml.api(
    openapi_overrides={
        "description": "Custom API description",
        "tags": ["custom-tag"],
        # Any valid OpenAPI field can be overridden
    }
)
def predict(self, input_data):
    ...

Link to Devin run: https://app.devin.ai/sessions/43f58b594b65481d81ffb5564bc6eb94

[devin-ai-integration]

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[github-actions]

Package	Statements
bentoml	55.17% (9912 / 17965)
Summary	55.17% (9912 / 17965)

[aarnphm]

Suggested change

	OpenAPI Specification Customization
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	OpenAPI Specification Customization
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^