Add license header to golang files

[giorio94]

Description

This PR adds a linting check to ensure a consistent license header is present in golang files, and adds it to all files.
@reviewers Probably the biggest PR ever, but an easy one. Either yes, or no :)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also note any relevant details for your test configuration.

• Checking the outcome of golangci-lint

[adamjensenbot]

Hi @giorio94. Thanks for your PR!

I am @adamjensenbot.
You can interact with me issuing a slash command in the first line of a comment.
Currently, I understand the following commands:

• /rebase: Rebase this PR onto the master branch (You can add the option test=true to launch the tests
  when the rebase operation is completed)
• /merge: Merge this PR into the master branch
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• /test Launch the E2E and Unit tests
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Make sure this PR appears in the liqo changelog, adding one of the following labels:

• kind/breaking: 💥 Breaking Change
• kind/feature: 🚀 New Feature
• kind/bug: 🐛 Bug Fix
• kind/cleanup: 🧹 Code Refactoring
• kind/docs: 📝 Documentation

[giorio94]

/test

[palexster]

/rebase test=true

[giorio94]

/test

[palexster]

/merge