Enhance finally block to support referenced stages

[vtselikovsky]

Hi there, could we consider implementing an option that allows including a reference to stages within the finally block?
This would enable us to achieve something similar to the following:

finally:
  - type: ref
    id: some_common_final_stage

tavern/tavern/_core/schema/tests.jsonschema.yaml

Line 511 in 7439490

$ref: "#/definitions/stage"

[michaelboulton]

Should be possible in 2.10.3