Packaging for elastic agent

[michalpristas]

What does this PR do?

This PR introduces a mage package command.
Result of this mage package command is an archive consisting of

• binary
• spec file
• config file

This archive is then consumed by elastic-agent and included in agent release artifact

Why is it important?

For agent to be able to work with shipper

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md or CHANGELOG-developer.md.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @michalpristas? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2022-09-27T07:22:39.925+0000

• Duration: 11 min 5 sec

❕ Flaky test report

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🤖 GitHub comments

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• /test : Re-trigger the build.

[leehinman]

be good to see if @faec has input before merging

[leehinman]

nit. We have been trying to get away from the "github.com/pkg/errors" package and just go with the std library "errors" package. Would you mind switching?

[cmacknz]

We should be able to just block this in the linter config: https://github.com/elastic/beats/blob/d777593494788495055fc5b0259613c33fb51e9a/.golangci.yml#L91-L101

For reference pkg/errors and the stdlib errors are almost but not quite compatible. See elastic/beats#31702.

[michalpristas]

yep

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b feat/packaging upstream/feat/packaging
git merge upstream/main
git push upstream feat/packaging

[cmacknz]

Confirmed the mage package target works locally for me, thanks!