Support additions to context made by adjust rules for follow-up rules

[happz]

adjust:
  - when: distro == rhel-9
    context+:
      llvm: 19

  - when: llvm == 19
    ...

Since fmf does not recognize context, this might need a combined effort of tmt and tmt, e.g. fmf accepting a callback updating context, or a list of keys affecting context...

No idea how messy this can get. Plus, I recall seeing similar questions in the past, there might be issues with discussions somewhere. Yet, it might be nice to give it more thought.

[kwk]

Just to give some context. This discussion came up here: https://src.fedoraproject.org/tests/lld/pull-request/5#c-6be99873e606937bdf7d3bbc29d42c98b8e9163f-26

[happz]

Filed teemtee/fmf#222 for the change, this would need to happen in fmf.