Add performance evaluators

[krisztianfekete]

Example config for testing:

evaluators:
  - name: token_efficiency
    type: code
    path: ../evaluators/evaluators/token_efficiency/token_efficiency.py
    threshold: 0.3
    config:
      max_input_tokens: 150000
      max_output_tokens: 50000

  - name: time_efficiency
    type: code
    path: ../evaluators/evaluators/time_efficiency/time_efficiency.py
    threshold: 0.3
    config:
      max_duration_s: 0.5
      latency_percentile: p95
      latency_source: overall

  - name: tool_efficiency
    type: code
    path: ../evaluators/evaluators/tool_efficiency/tool_efficiency.py
    threshold: 0.5
    config:
      max_tool_calls: 3
      min_tool_calls: 2
      penalize_duplicates: true
      penalize_errors: true

Supersedes #7
Fixes #6

[peterj]

minor nits