Purpose of this pull request

Which issue you fix

Add Apache 2.0 license.

Checklist:

• I have executed the 'mvn spotless:apply' command to format my code.
• I have a meaningful commit message (including the issue id, the template of the commit message is '[label-type-#issue-id][fixed-module] a meaningful commit message.')
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have checked my code and corrected any misspellings.
• My commitment is only one. (If there are multiple commits, you can use 'git squash' to compress multiple commits into one.)