digraph: Support JSON reference expansion

[yohamta]

Implemented syntax for JSON reference in variables to dynamically expand values from JSON variables.

Example:

steps:
  - name: sub workflow
    run: sub_workflow
    output: SUB_RESULT
  - name: use output
    command: echo "The result is ${SUB_RESULT.outputs.finalValue}"
    depends:
      - sub workflow

If SUB_RESULT contains:

{
  "outputs": {
    "finalValue": "success"
  }
}

Then the expanded value of ${SUB_RESULT.outputs.finalValue} will be success. It works for any environment variables and params as well.

[codecov]

Codecov Report

Attention: Patch coverage is 67.55448% with 134 lines in your changes missing coverage. Please review.

Project coverage is 55.70%. Comparing base (bc71e96) to head (5efc380).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/cmdutil/eval.go	58.09%	80 Missing and 8 partials ⚠️
internal/cmdutil/cmd.go	66.10%	29 Missing and 11 partials ⚠️
internal/digraph/scheduler/node.go	40.00%	2 Missing and 1 partial ⚠️
internal/digraph/step_context.go	0.00%	2 Missing ⚠️
internal/digraph/builder.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #778      +/-   ##
==========================================
+ Coverage   55.19%   55.70%   +0.50%     
==========================================
  Files          71       73       +2     
  Lines        7712     7825     +113     
==========================================
+ Hits         4257     4359     +102     
- Misses       3082     3091       +9     
- Partials      373      375       +2     

Files with missing lines	Coverage Δ
cmd/dry.go	68.00% <100.00%> (ø)
cmd/restart.go	53.57% <100.00%> (ø)
cmd/retry.go	54.05% <100.00%> (ø)
cmd/setup.go	75.82% <100.00%> (-0.27%)	⬇️
cmd/start.go	62.72% <100.00%> (ø)
internal/cmdutil/sub.go	100.00% <100.00%> (ø)
internal/digraph/condition.go	52.22% <100.00%> (ø)
internal/digraph/context.go	26.66% <100.00%> (ø)
internal/digraph/params.go	69.75% <100.00%> (ø)
internal/digraph/variables.go	74.46% <100.00%> (ø)
... and 5 more

... and 2 files with indirect coverage changes

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