feat: attribute referencing and expression support

[mathnogueira]

This PR allows users to specify assertions using variables from the selected span and to use basic arithmetic expressions with it.

Examples:

testDefinition:
  selector: span[name = "proxy"]
  assertions:
    - http.status_code = http.proxy.status_code # referencing an attribute from the span
    - tracetest.span.endtime < tracetest.span.startTime + 800ms # same as duration < 800ms

Checklist

• [x] tested locally
• [ ] added new dependencies
• [ ] updated the docs
• [x] added a test

[mathnogueira]

Related to #782

[mathnogueira]

@schoren applied changes.