Support service account impersonation with BigQuery

[k1LoW]

ref: #360

[github-actions]

Code Metrics Report

	main (072711d)	#363 (ba224b5)	+/-
Coverage	67.2%	64.7%	-2.5%
Code to Test Ratio	1:0.5	1:0.5	-0.0
Test Execution Time	3m1s	2m20s	-41s

Details

  |                     | main (072711d) | #363 (ba224b5) |  +/-  |
  |---------------------|----------------|----------------|-------|
- | Coverage            |          67.2% |          64.7% | -2.5% |
  |   Files             |             33 |             33 |     0 |
  |   Lines             |           3160 |           3171 |   +11 |
- |   Covered           |           2123 |           2051 |   -72 |
- | Code to Test Ratio  |          1:0.5 |          1:0.5 |  -0.0 |
  |   Code              |           8236 |           8250 |   +14 |
  |   Test              |           3887 |           3887 |     0 |
+ | Test Execution Time |           3m1s |          2m20s |  -41s |

Code coverage of files in pull request scope (38.6% → 10.6%)

Files	Coverage	+/-
datasource/gcp.go	10.6%	-27.9%

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Reported by octocov