build: format pom and code

[tisonkun]

Before starting hacking in this project, employing common code formatting rules:

• License header checker makes easy.
• Spotless for auto formatting.
• Sortpom can fail on verify; properties group helps.
• Remove redundant properties.
• Drop @author tag to avoid "this is not my file" feeling.

[tisonkun]

Some follow-ups -

• Add docs about how to run the formatter (spotless, hawkeye)
• Try to integrate formatting into compile phase or the default lifecycle