Add Licenser and license headers

[jlvoiseux]

Motivation / Summary

This PR adds the go-licenser module to the codebase. The module is then used as a preparatory step before builds : it checks for the presence of the Apache v2 license header in all Go files in the project, and adds/updates it if it absent / not compliant to Elastic's standards.

Resolves #47.

[ghost]

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Build stats

• Start Time: 2022-03-28T13:28:25.200+0000

• Duration: 15 min 51 sec

Test stats 🧪

Test	Results
Failed	0
Passed	240
Skipped	6
Total	246

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[estolfo]

I think this needs to change before merging

[jlvoiseux]

Solved in 4b24c14.