[infra] Improve Fuzz-Introspector support

[renatahodovan]

This patch enhances introspector by adding engine-specific and environment variable arguments. This allows performance measurement not only for libFuzzer but also for other fuzzing engines.

[renatahodovan]

I've been experimenting with different fuzzing engines and wanted to compare their results using Fuzz-Introspector. However, I couldn't find a built-in way to do this, even though the implementation supports it—key parameters just aren’t exposed via the CLI. This patch addresses that gap. What do you think about adding this feature? Additionally, the environment arguments could potentially be split into separate build-time and runtime arguments for further refinement.

[jonathanmetzman]

@DavidKorczynski Could you help review this please?