feat: add quiet input that maps to --quiet

[szeyu]

This pull request introduces support for a new quiet input, allowing users to suppress build output by mapping to the --quiet flag. The changes update the documentation, action input definitions, input parsing, and argument construction to support this new feature.

Support for quiet build option:

• Added quiet input to action.yml with description and default value, enabling users to suppress build output.
• Documented the new quiet input in the README.md table of inputs.

Code updates for input handling:

• Added quiet property to the Inputs interface in src/context.ts.
• Updated getInputs() in src/context.ts to parse the quiet input as a boolean.
• Modified getCommonArgs() in src/context.ts to append --quiet to build arguments if the input is set.

[crazy-max]

Thanks for your contrib but similar to:

• Add support for --progress CLI option #1092
• Plain output for buildx ? #1014
• Enable Buildx progress=plain #885 (comment)

We don't want to provide explicit input as it can be done through BUILDKIT_PROGRESS env var. In your case you can do:

      -
        name: Build and push
        uses: docker/build-push-action@v6
        with:
          push: true
          tags: user/app:latest
        env:
          BUILDKIT_PROGRESS: quiet

You also have none available since Buildx 0.29 that doesn't print the image ID.