Expose gcp.resourceLabels field in the gcp plugin

[pirrofra]

What type of PR is this?
/kind feature

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

What this PR does / why we need it:
On the GCP Plugin we already expose the resource type via the gcp.resourceType field. Given that the log-entry also supports a labels field which can contains useful information for correctly identifying a resource on GCP, we want to expose that using a new gcp.resourceLabels field.
This field will contain the JSON representation of the labels attached on the cloud log entry.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[poiana]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: pirrofra
Once this PR has been reviewed and has the lgtm label, please assign ahmedameenaim for approval. For more information see the Kubernetes Code Review Process.

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

Needs approval from an approver in each of these files:

• plugins/gcpaudit/OWNERS

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[poiana]

Welcome @pirrofra! It looks like this is your first PR to falcosecurity/plugins 🎉

[SKosier]

Two small comments:

Please bump the version of the plugin here
And add the new extraction to the README

[github-actions]

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Comparing dedac44e0f2e17c38450f011d53ebb9c5c371a26 with latest tag plugins/gcpaudit/v0.4.0

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[github-actions]

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Comparing c725709ab08ac31579161385861ea3cd10554660 with latest tag plugins/gcpaudit/v0.4.0

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Comparing 0f3b7eb1016aad6f548fa10983802c4636142b0e with latest tag plugins/gcpaudit/v0.4.0

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