[#755] dag: Support params as a list of key-value pairs

[yohamta]

Resolves #755

Example:

params:
  - FOO: foo
  - BAR: bar
  - BAZ: "`echo baz`"
steps:
  - name: step_1
    command: |
      echo ${FOO} ${BAR} ${BAZ}

[codecov]

Codecov Report

Attention: Patch coverage is 75.38462% with 32 lines in your changes missing coverage. Please review.

Project coverage is 58.29%. Comparing base (b151210) to head (b9e3d08).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/digraph/params.go	75.38%	25 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #756      +/-   ##
==========================================
+ Coverage   58.11%   58.29%   +0.18%     
==========================================
  Files          68       68              
  Lines        6640     6729      +89     
==========================================
+ Hits         3859     3923      +64     
- Misses       2474     2495      +21     
- Partials      307      311       +4     

Files with missing lines	Coverage Δ
internal/digraph/params.go	80.45% <75.38%> (-8.96%)	⬇️

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