feat: gradio integration

[parano]

What does this PR address?

This PR simplifies how users can serve Gradio UI for their BentoML service. Previously user will need to manually do multiple settings and callbacks, e.g.:

import bentoml
import gradio as gr

def summarize_text(text: str) -> str:
    svc_instance = bentoml.get_current_service()
    return svc_instance.summarize([text])[0]

io = gr.Interface(
    fn=summarize_text,
    inputs=[gr.Textbox(lines=5, label="Enter Text", value=EXAMPLE_INPUT)],
    outputs=[gr.Textbox(label="Summary Text")],
    title="Summarization",
    description="Enter text to get summarized text."
)
gradio_app = gr.routes.App.create_app(io)

io.dev_mode = False
io.show_error = True
io.validate_queue_settings()

@bentoml.mount_asgi_app(gradio_app, path="/ui")
@bentoml.service(resources={"cpu": "4"})
class Summarization:

    def __init__(self) -> None:
        import torch
        from transformers import pipeline

        # gradio_app.get_blocks().queue()
        gradio_app.get_blocks().startup_events()
        # gradio_app.get_blocks().root_path="/ui"

        ...

With this PR, this can be simplified as:

def summarize_text(text: str) -> str:
    svc_instance = bentoml.get_current_service()
    return svc_instance.summarize([text])[0]

io = gr.Interface(
    fn=summarize_text,
    inputs=[gr.Textbox(lines=5, label="Enter Text", value=EXAMPLE_INPUT)],
    outputs=[gr.Textbox(label="Summary Text")],
    title="Summarization",
    description="Enter text to get summarized text."
)

@bentoml.service(resources={"cpu": "4"})
@bentoml.gradio.mount_gradio_app(io, path="/ui")
class Summarization:
      ...

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[aarnphm]

probably want frost opinion on this:

For any integration going forward, should it be under bentoml._internal or _bentoml_sdk?

I think it should be the second option?