feature: deferred loading and requirement pruning

[leondz]

Support construction-time loading of optional modules. Includes

• many generators now using this pattern
• pattern for loading modules at run-time and failing if absent
• optional requirements moved to pyproject.toml options and pruned from requirements.txt
• _load_deps and _clear_deps pattern, used in generator constructor and _load_client / _clear_client
• tests to check that optional deps are in the right place and not in the wrong place

Todo / in scope:

• skipping tests requiring absent optional imports
• take a stance on whether to skip or stop on missing module, per plugin type (likely skip for probe, stop for main generator, beyond that tba)
• Consider moving load/clear dep function defs up and out for all classes (where - garak._plugins? How - assign func member? create mixin?)
• validation: exception throws correctly / doesn't throw if dependency present; module still runs
  • generators.cohere
  • generators.langchain
  • generators.litellm
  • generators.mistral
  • generators.nemoguardrails
  • generators.nemollm
  • generators.ollama
  • generators.optimum (huggingface.OptimumPipeline) - exception fires, pkg install seems borked
  • generators.replicate
  • probes.audio

Not done:

• load/clear deps for plugin types other than generators
• gh actions for testing optional components
• deeper validation

Out of scope:

• handling of versioning outside of pyproject.toml

Resolves #101

[leondz]

need to solve how optional/deferred testing works