Allow optional _dev/tools directory in integration packages for development tooling files.

[marc-gr]

What does this PR do?

Allow optional _dev/tools directory in integration packages for development tooling files.

Why is it important?

We have some cases where tooling is needed in order to generate files etc as part of a package development. This change would allow us to have them in the package instead of multiple repositories

Checklist

• I have added test packages to test/packages that prove my change is effective.
• I have added an entry in spec/changelog.yml.

[elasticmachine]

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• Commit: cd5a275

[mrodm]

What kind of scripts or files do you expect to be placed in the new _dev/tools folder?

I found a related issue in elastic-package that seems to address a similar purpose, along with some relevant discussion: elastic/elastic-package#950

cc @jsoriano @teresaromero

[marc-gr]

For osquery_manager we need to run scripts to generate field files and other schemas, I would also be fine with _dev/scripts if we prefer to go that way, also

[teresaromero]

i think we can address both issues with this change. I prefer calling it scripts. However, we could be opening a way to create tooling under the packages, instead of addressing the need at elastic-package as it is mentioned at elastic/elastic-package#950 (comment)