Source Code and license formatting

This PR introduce spotless maven plugin to manage:

• license headers
• java source code formatting (via eclipse formatter)
• pom formatting

Java and Pom formatting are not yet enabled because we need to get the licenses right. According to https://git.eliatra.com/eliatra-suite/eliatra-opensearch-plugin/-/issues/29 the spotless plugin does

• Apply ee license to all ee projects (overwriting the old licenses)
• Apply aslv2 to all non-ee projects (prepend new eli license header)

This should work automatically after a one time sanitization step:

• Run ./dev/fix_headers.sh (can later be deleted if everything is ok)
  • This add missing aslv2 fg headers to non-ee projects
  • Add a comment "//== Original license headers ==" above the original license for non-ee projects (spotless needs this so that original license gets not overrridden)
• Run mvn spotless:apply
• Find code which we probably want to exclude from formatting and annotate it
• Check everything - if all is ok commit, and delete ./dev/fix_headers.sh