[RNDSTROPPY-72]: k6 without subprocesses

[Cianidos]

Description of Changes

K6 extension with subcommand

Now Stroppy is both a k6 subcommand and a stand-alone CLI.
Stroppy acts differently based on the binary file name:
./stroppy ... - acts like the good old Stroppy.
mv stroppy k6 && ./k6 ... - acts like k6 with the Stroppy extension.

Get rid of k6 subprocess

Now k6 starts directly as a library.
It still requires preparing environment variables and args, but no processes are spawned.
Also, the embedded stroppy-k6 binary was removed.

Signals confirmation mechanic

Interrupt signal, aka Ctrl+C, is processed for explicit interactive confirmation.
The user should press Ctrl+C twice within a 5-second window.

xk6 sync

Now dependencies of the Stroppy and xk6air modules are synchronized with k6 dependency versions via xk6 sync.

Makefile, Dockerfile, CI...

Updated

helpers

Missed metric fix.

Motivation and Context

Why is this change required?

How Has This Been Tested?

Describe tests or steps to verify.

Type of Changes

• Bug fix
• New feature
• Documentation improvement
• Refactoring
• Other

Checklist

• I have read the CONTRIBUTING.md
• I have checked build and tests
• I have updated documentation if needed