Add bind runner for binding variables

[k1LoW]

steps:
  - req: 
      /users/k1low:
        get:
          body: null
  - bind:
      user_id: steps[0].res.body.data.id
  - dump: user_id

[github-actions]

Code Metrics Report

	main (5e4305e)	#30 (3cf117e)	+/-
Coverage	71.9%	70.3%	-1.6%
Code to Test Ratio	1:0.7	1:0.7	+0.0
Test Execution Time	31s	25s	-6s

Details

  |                     | main (5e4305e) | #30 (3cf117e) |  +/-  |
  |---------------------|----------------|---------------|-------|
- | Coverage            |          71.9% |         70.3% | -1.6% |
  |   Files             |             10 |            11 |    +1 |
  |   Lines             |            707 |           740 |   +33 |
+ |   Covered           |            508 |           520 |   +12 |
+ | Code to Test Ratio  |          1:0.7 |         1:0.7 |  +0.0 |
  |   Code              |           1513 |          1580 |   +67 |
+ |   Test              |           1017 |          1103 |   +86 |
+ | Test Execution Time |            31s |           25s |   -6s |

Code coverage of files in pull request scope (69.7% → 65.7%)

Files	Coverage	+/-
bind.go	80.0%	+80.0%
operator.go	65.2%	-4.6%

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Reported by octocov