refactor: Use string_view in event filters and more

[federico-sysdig]

What type of PR is this? /kind cleanup

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

Does this PR require a change in the driver versions?

What this PR does / why we need it: Some refactorings to try and improve the performance of event filtering. In particular the PR concentrates on avoiding string creation by passing around only std::string_view objects. Some minor improvements have been also applied, such as creating objects controlled by a shared_ptr with std::make_shared to ensure that the control block is not allocated separate from the object, thus avoiding one allocation and making it more cache friendly.

Which issue(s) this PR fixes: Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[FedeDP]

Since we are a bit late for the 0.16.0 tag, i propose: /milestone 0.17.0

[leogr]

/assign @FedeDP :pray:

[leogr]

cc @jasondellaluce

[poiana]

LGTM label has been added.

Git tree hash: 225128f4dc00cf1a63bb56b4536a9d3039f02a77

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: FedeDP, federico-sysdig, incertum

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