Cleanup warnings and smart ptrs

[federico-sysdig]

What type of PR is this?
/kind cleanup

Any specific area of the project related to this PR?
/area build

What this PR does / why we need it:
This is just a cleanup/refactor on some usages of smart pointers, plus a fix for a warning about a pessimizing std::move raised by Clang.
The changes might seem very diverse, but they are very simple and arranged into separate commits if the reviewer prefers to address the task for each topic separately.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[github-actions]

This PR may bring feature or behavior changes in the Falco engine and may require the engine version to be bumped.

Please double check userspace/engine/falco_engine_version.h file. See versioning for FALCO_ENGINE_VERSION.

/hold

[jasondellaluce]

/approve

[poiana]

LGTM label has been added.

Git tree hash: 2046d5313decbb1d001fbc7a629c424e8f638891

[FedeDP]

/milestone 0.38.0
/hold
Since ci is failing.

[FedeDP]

/hold

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: federico-sysdig, jasondellaluce, LucaGuerra

The full list of commands accepted by this bot can be found here.

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Needs approval from an approver in each of these files:

• OWNERS [LucaGuerra,jasondellaluce]

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[LucaGuerra]

Thanks @FedeDP ! Let's see why the CI has some problems (might be a job config)

[FedeDP]

/unhold
CI is unrelated.