RFR: 7449: Add GitHub action to verify copyright year

[Alex Ciminian]

This PR adds a script to check copyright notices on changed files. I've not added a GitHub action yet, I'd like to get some feedback before doing that �� 

The script is written in Kotlin and uses [JGit](https://www.eclipse.org/jgit/) to interface with the git repository. To run, it only needs the kotlin compiler which is [preinstalled on GitHub Actions runners](https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-README.md).

It works as follows:

* diff the HEAD commit and the merge base between it and master (i.e. where the feature branch diverged from)
* for every changed file (except deletions), validate the copyright header
  - if the file is new, it should have Oracle and Datadog copyright starting from the year of the HEAD commit
  - if the file is not new, it should keep the existing copyright holders and update the year range if necessary
* we compare file contents from objects in the git repo, not the files themselves
* currently we don't validate we're in the correct directory or the commits

The output looks like this:


kotlinc -script scripts/copyrightchecker.main.kts                                                                                                                                                     
Checked out at: 759788185939b16f447e553b3ee64121187ea311 (base: 8b8aa7c299b92b655ba669a92d16f193beb406bc)
⚪️ scripts/copyrightchecker.main.kts
✅ Validation succeeded



kotlinc -script scripts/copyrightchecker.main.kts                                                                                                                                                       
Checked out at: 7ad43eed12c9eb85a96882c9518a9acbe12e74c5 (base: ca204424ce32336f2923aff66f619d571b1fc720)
⚪️ .gitignore
�� application/org.openjdk.jmc.feature.flightrecorder/feature.xml
�� Failed to parse:    Copyright (c) 2018, 2021 Oracle and/or its affiliates. All rights reserved.
⚪️ application/org.openjdk.jmc.flightrecorder.heatmap/.classpath
⚪️ application/org.openjdk.jmc.flightrecorder.heatmap/.gitignore
⚪️ application/org.openjdk.jmc.flightrecorder.heatmap/META-INF/MANIFEST.MF
⚪️ application/org.openjdk.jmc.flightrecorder.heatmap/build.properties
⚪️ application/org.openjdk.jmc.flightrecorder.heatmap/icons/heatmap.png
⚪️ application/org.openjdk.jmc.flightrecorder.heatmap/icons/heatmap at 2x.png
⚪️ application/org.openjdk.jmc.flightrecorder.heatmap/plugin.properties
⚪️ application/org.openjdk.jmc.flightrecorder.heatmap/plugin.xml
�� application/org.openjdk.jmc.flightrecorder.heatmap/pom.xml
Expected:
<?xml version="1.0" encoding="UTF-8"?>
<!--
	Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
	Copyright (c) 2021, Datadog, Inc. All rights reserved.

	DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
Actual:
<?xml version="1.0" encoding="UTF-8"?>
<!--
   Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
   Copyright (c) 2021, Datadog, Inc. All rights reserved.

   DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HE
(check whitespace if strings seem to match)
�� application/org.openjdk.jmc.flightrecorder.heatmap/src/main/java/org/openjdk/jmc/flightrecorder/heatmap/views/HeatmapView.java
�� application/org.openjdk.jmc.flightrecorder.heatmap/src/main/resources/heatmap.js
⚪️ application/org.openjdk.jmc.flightrecorder.heatmap/src/main/resources/page.template
�� application/pom.xml
Expected:
<?xml version="1.0" encoding="UTF-8"?>
<!--
	Copyright (c) 2018, 2021, Oracle and/or its affiliates. All rights reserved.

	DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
Actual:
<?xml version="1.0" encoding="UTF-8"?>
<!--
   Copyright (c) 2018, 2021, Oracle and/or its affiliates. All rights reserved.

   DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEAD
(check whitespace if strings seem to match)

❌ Validation failed

-------------

Commit messages:
 - Add script to check copyright header on changed files

Changes: https://git.openjdk.java.net/jmc/pull/342/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=342&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7449
  Stats: 285 lines in 1 file changed: 285 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/342.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/342/head:pull/342

PR: https://git.openjdk.java.net/jmc/pull/342