Enable auto license checking

[jihoonson]

This patch contains a bunch of changes for license headers of java files. Key changes are:

• Added LICENSE.txt to codestyle directory.
• Added <headerLocation>codestyle/LICENSE.txt</headerLocation> to {project.root}/pom.xml
• Added the following contents to checkstyle.xml

<module name="Header">
    <property name="headerFile" value="${checkstyle.header.file}"/>
    <property name="fileExtensions" value="java"/>
  </module>

• Added <suppress checks="Header" files="[\\/]target[\\/]generated-test-sources[\\/]" /> to checkstyle-suppressions.xml

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This change is 

[gianm]

Thanks @jihoonson, more checkstyles are more better!

I reviewed the changes ignoring whitespace (https://github.com/druid-io/druid/pull/3836/files?w=1) and it looks like other than whitespace, mostly you are adding headers to files that miss them, and changing the old style headers in druid-api to new style headers. That's good, since we just moved them in a while back without updating the headers.

Could you please fix the headers on SQLServerConnector.java, BoundDimFilterTest.java, SQLMetadataRuleManager.java, and SQLMetadataSegmentPublisher.java? They all have two headers now.

After that the change LGTM

[fjy]

👍 from me after addressing @gianm's comment

[jihoonson]

@gianm, @fjy thank you for your review. I fixed it.

[gianm]

👍 LGTM