Enable selectors to utilize variables

[kdhamric]

As a user, I may have selector names that include data based on ids or other dynamic data and want to be able to have a selector that incorporates variables, interprets them, and utilizes the final string as the selector.

A good way to see / reproduce this issue is to use the following test in our tracetest-demo org in the pokeshop-demo environment (join here). It takes the pokeshop_id as a parameter. In the selector, I need to be able to use the pokemon_id as a variable and not have it hardcoded (ie this needs to be dynamic based on the pokemon_id: name="del pokemon-9693")

Priority: low to medium - do as we have cycles as this came from an internal use case.

type: Test
spec:
 id: delete-pokemon
 name: Delete Pokemon
 trigger:
   type: http
   httpRequest:
     method: DELETE
     url: https://demo-pokeshop.tracetest.io/pokemon/${env:pokemon_id}
     headers:
     - key: Content-Type
       value: application/json
 specs:
 - selector: span[tracetest.span.type="general" name="Tracetest trigger"]
   name: Delete returns a 200 status code
   assertions:
   - attr:tracetest.response.status = 200
 - selector: span[tracetest.span.type="database" name="del pokemon-9693" db.system="redis" db.operation="del" db.redis.database_index="0"]
   name: Ensure we are deleting from the redis cache also
   assertions:
   - attr:tracetest.selected_spans.count = 1