appsec: handle user custom rules through remote config

[Hellzy]

What does this PR do?

This allows users to send custom security rules through the ASM remote config product. Custom rules
are processed by the WAF just like regular rules, but are described under a different json field in the rule file (custom_rules). Custom rules can be present along regular rules.

Motivation

Describe how to test/QA your changes

Reviewer's Checklist

• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[pr-commenter]

Benchmarks

Comparing candidate commit 238bd32 in PR branch francois.mazeau/custom-rules with baseline commit 242cc76 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 18 metrics, 0 unstable metrics.