Add user agent when default_user_agent_disabled is false

[3dgiordano]

This pull request enhances how the HTTP/2 plugin sets the User-Agent header by introducing logic to add a default User-Agent when none is configured, unless explicitly disabled via a property. It also adds comprehensive tests to verify this behavior, ensuring compatibility with legacy behavior and configurability.

User-Agent header handling improvements:

• Added logic in JmeterRequestHeadersSupport to automatically set a default User-Agent header ("BlazeMeter HTTP" or "BlazeMeter HTTP/<version>") if none is provided, unless the httpclient4.default_user_agent_disabled property is set to true. This matches the legacy behavior of HTTPHC4Impl and improves interoperability. [1] [2]
• Introduced helper methods and constants in JmeterRequestHeadersSupport to support the new default User-Agent logic, including property checks and default value construction. [1] [2]

Testing enhancements:

• Added new tests in JmeterRequestHeadersSupportTest to verify that the default User-Agent is set when missing, not overridden if already present (including empty values), and omitted when the disabling property is set.
• Improved test setup and teardown by clearing the relevant property after each test to avoid side effects.

Dependency and import updates:

• Added necessary imports for JMeterUtils, StringUtils, and test utilities to support the new logic and tests. [1] [2]