PoC - Trace test variables

[perlpunk]

This is just a proof of concept.

Issue: https://progress.opensuse.org/issues/156697

Example output:

[2024-03-05T14:53:02.149581+01:00] [debug] [pid:23635] ---
  load_vars:
    ARCH: x86_64
    # ...
  override:
    DUMMY:
      callstack:
      - [boot, 7, testapi::set_var]
      - [basetest, 352, boot::run]
      value: lala
    NEEDLES_GIT_HASH:
      callstack:
      - [needle, 292, bmwqemu::tiedvars::STORE]
      - [OpenQA::Isotovideo::Runner, 72, needle::init]
      value: f39035e15d5d7535b6bc66bbcd9cde565a978945
    NEEDLES_GIT_URL:
      callstack:
      - [needle, 292, bmwqemu::tiedvars::STORE]
      - [OpenQA::Isotovideo::Runner, 72, needle::init]
      value: [email protected]:os-autoinst/os-autoinst-needles-openQA.git
    TEST_GIT_HASH:
      callstack:
      - [OpenQA::Isotovideo::Runner, 215, bmwqemu::tiedvars::STORE]
      - [OpenQA::Isotovideo::Runner, 68, OpenQA::Isotovideo::Runner::checkout_code]
      value: 552780e98a3a1d4881f4b42b6d381848cb980837
    TEST_GIT_URL:
      callstack:
      - [OpenQA::Isotovideo::Runner, 215, bmwqemu::tiedvars::STORE]
      - [OpenQA::Isotovideo::Runner, 68, OpenQA::Isotovideo::Runner::checkout_code]
      value: [email protected]:os-autoinst/os-autoinst-distri-openQA