WIP: HTTP parity regression

[3dgiordano]

This pull request introduces a comprehensive JMeter HTTP regression parity test suite, updates dependencies, and improves project configuration for testing and code quality. The main focus is on enabling automated regression testing to compare the BlazeMeter HTTP sampler with Apache JMeter's HttpClient4 using official test plans, while also modernizing dependency versions and build tooling.

Key changes include:

Regression Testing Infrastructure:

• Adds a new regression test suite for HTTP sampler parity, including a detailed documentation file (docs/jmeter-regression.md) explaining the tiers, test plans, and usage. This suite automates comparison between BlazeMeter and Apache JMeter HTTP samplers using official JMeter 5.6.3 test plans. [1] [2]
• Updates the CI workflow (ci-jmeter-compatibility.yaml) to run all regression tiers in CI, ensuring comprehensive compatibility coverage.
• Introduces Maven profiles (jmeter-regression, jmeter-regression-f4, jmeter-regression-flaky) in pom.xml to support different regression tiers and test sets, with flexible system property configuration.

Dependency and Build Updates:

• Bumps JMeter dependencies to version 5.6.3 and updates related libraries (HttpClient, commons-io, commons-lang3), ensuring compatibility with the latest JMeter release and test plans. [1] [2] [3]
• Upgrades the Maven Checkstyle plugin and its dependencies, and moves the LineLength rule to the top-level in checkstyle.xml for better code style enforcement. [1] [2] [3]

Project Configuration and Cleanliness:

• Adds a .gitattributes file to enforce consistent line endings for all relevant file types, improving cross-platform collaboration.
• Removes unused imports and dead code from HTTP2FutureResponseListener, simplifying the codebase. [1] [2] [3] [4] [5]

These changes collectively enhance the project's test coverage, maintainability, and alignment with modern JMeter and Java development practices.