feature: disable optional imports by default

[leondz]

Reduce garak default footprint

depends on #1199

This PR tracks work to reduce garak's footprint by making many libraries optional

• remove requirements.txt
• move optional imports to clauses
• update .github/ to use non-requirements.txt routes
• update external documentation
• ensure test_reqs ises intent at the commit this branch stems from, and the regressions in the parent branch are not merged here
• check tests represent desired behaviour
• check pyproject.toml format is as desired
• update developer documentation
• update in-repo getting started guide
• validate documentation
• check that 3.10 supports this as we're doing it - or make min ver 3.11
• check that pypi supports intended behaviour
• work out how we want runs to behave if they're summoning a required but missing module (early quit)
  • .. and test this