Add runbookID() for returning the runbook ID of the root runbook.

[k1LoW]

ref: #645 (comment)

[k1LoW]

@k2tzumi operator.runbookIDFull() and step.runbookIDFull() can be used to get the id needed for trace.

[github-actions]

Code Metrics Report

	main (2a52af3)	#646 (d2e78c5)	+/-
Coverage	69.5%	69.7%	+0.2%
Code to Test Ratio	1:0.7	1:0.7	-0.0
Test Execution Time	5m8s	2m47s	-2m21s

Details

  |                     | main (2a52af3) | #646 (d2e78c5) |  +/-   |
  |---------------------|----------------|----------------|--------|
+ | Coverage            |          69.5% |          69.7% |  +0.2% |
  |   Files             |             51 |             51 |      0 |
  |   Lines             |           5546 |           5582 |    +36 |
+ |   Covered           |           3852 |           3888 |    +36 |
- | Code to Test Ratio  |          1:0.7 |          1:0.7 |   -0.0 |
  |   Code              |          11171 |          11249 |    +78 |
+ |   Test              |           7711 |           7750 |    +39 |
+ | Test Execution Time |           5m8s |          2m47s | -2m21s |

Code coverage of files in pull request scope (77.0% → 77.9%)

Files	Coverage	+/-
operator.go	77.1%	+0.6%
step.go	95.7%	-0.9%
trail.go	70.0%	0.0%

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Reported by octocov

[k2tzumi]

@k1LoW
Great!
LGTM!