CI Overhaul

[pingsutw]

• Speed up CI/CD pipeline by 20x
• Use uv to install the packages
• Remove requirements.txt since the majority of them are still using a pretty old flytekit (<1.0)
• Build and push ImageSpec. (we didn't build imageSpec before)
• Instead of serializing the example inside a Docker container, serialize it locally
• Remove the push_example_image_to_github workflow since it hasn't been used anywhere. In addition, we're using ImageSpec in the flytesnacks example, so it doesn't make sense to build an image from the Dockerfile
• Serailzie and register all the examples under examples folder.

Follow-up PR

• Remove Dockerfile