Extend exists operator to support key paths and negation

[Anilm3]

This PR adds support for asserting the presence of key paths when using the exists operator, such as through the following example:

  - id: 2
    name: rule2-exists-kp
    tags:
      type: flow
      category: category
    conditions:
      - operator: exists
        parameters:
          inputs:
            - address: input-2
              key_path: ["path"]

In addition, the !exists operator has also been introduced. Since addresses are provided in stages, it's not currently possible to safely assert the absence of an address, hence the operator can only be used with key paths, and limited to a single address, for example:

  - id: 2
    name: rule2-not-exists-kp
    tags:
      type: flow
      category: category
    conditions:
      - operator: "!exists"
        parameters:
          inputs:
            - address: input-2
              key_path: ["path"]

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.75510% with 6 lines in your changes missing coverage. Please review.

Project coverage is 84.61%. Comparing base (aeca2d0) to head (a5d8f5d).

Files with missing lines	Patch %	Lines
src/condition/exists.cpp	85.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
+ Coverage   84.56%   84.61%   +0.05%     
==========================================
  Files         147      148       +1     
  Lines        7037     7080      +43     
  Branches     3252     3271      +19     
==========================================
+ Hits         5951     5991      +40     
- Misses        409      411       +2     
- Partials      677      678       +1     

Flag	Coverage Δ
waf_test	84.61% <87.75%> (+0.05%)	⬆️

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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-13 16:46:06

Comparing candidate commit e602c9a in PR branch anilm3/negated_exists with baseline commit aeca2d0 in branch master.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 0 metrics, 0 unstable metrics.

scenario:global-benchmark.random

• 🟩 execution_time [-14.304ms; -14.239ms] or [-4.940%; -4.918%]

[cataphract]

Looks good, you can mayb