Update license to Apache License Version 2.0

[qingyun-wu]

Why are these changes needed?

Summary

This pull request changes the project license from MIT to Apache 2.0 for all new and modified code, while preserving the MIT license for original code.

Changes Made

• Added new LICENSE file with Apache 2.0 license text
• Renamed original MIT license to LICENSE_original_MIT and included in the folder license_original
• Added NOTICE file explaining the dual licensing
• Updated README.md to reflect license changes
• Updated file headers in modified files to Apache 2.0
• Updated the contributing guide to give contributors instructions

Rationale

The Apache 2.0 license was chosen for the following reasons:

• It provides explicit patent rights, which offers better protection for contributors and users
• It's compatible with many other open-source licenses, including MIT
• It's widely recognized and trusted in the open-source community

Compliance

• Original MIT-licensed code remains under MIT license (the original license is included in the license_original/LICENSE_original_MIT file)
• All new and modified code will be under Apache 2.0
• Dual licensing is clearly explained in README and NOTICE files

Next Steps

• Review and merge this PR
• Update project metadata on GitHub to reflect the license change
• Notify existing contributors and users about the license change

Related issue number

Checks

• I've included any doc changes needed for https://autogen-ai.github.io/autogen/. See https://autogen-ai.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[Hk669]

looks good to me. thanks for the efforts in updating the license, really appreciated @qingyun-wu