feat: Add AIX support for system/filesystem

[djboris9]

What does this PR do?

It implements the filesystem listing on AIX in system/filesystem. This is done by calling the mntctl function and processing the returned vmount structures. There is nothing such as /proc/self/mounts available on AIX and therefore I had to go with CGo calling the same functions as mount or df does.

Why is it important?

This extends the AIX support for this package and metricbeat will benefit of it.
When this is merged, I will also create a PR to enable the according modules (system/filesystem, system/fsstat) in metricbeat for AIX.

Checklist

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

Author's Checklist

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• Start Time: 2022-11-05T23:12:18.745+0000

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[djboris9]

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[nimarezainia]

@djboris9 thanks for this PR. Unfortunately at the moment we don't have the facility to test and ensure these beats are functioning correctly on AIX. We will keep this PR for such time when that is possible.

[serak]

@djboris9 thanks for this PR. Unfortunately at the moment we don't have the facility to test and ensure these beats are functioning correctly on AIX. We will keep this PR for such time when that is possible.
we just need a successful cross compilation