internal/appsec: fix derivatives serdes on simple types

[eliottness]

What does this PR do?

This PR fixes fingerprinting derivatives received from the WAF that were serialized in json as string, hence adding another set of quotes to the values

Motivation

Support for Attacker Fingerprinting

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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