Support external database driver

[k1LoW]

If the executable tbls-driver-foodb is on the PATH, tbls will recognize the foodb:// scheme.

[github-actions]

Code Metrics Report

	main (576d46c)	#649 (3e16226)	+/-
Coverage	55.9%	54.1%	-1.9%
Code to Test Ratio	1:0.4	1:0.4	-0.1
Test Execution Time	4m29s	4m29s	0s

Details

  |                     | main (576d46c) | #649 (3e16226) |  +/-  |
  |---------------------|----------------|----------------|-------|
- | Coverage            |          55.9% |          54.1% | -1.9% |
  |   Files             |             52 |             52 |     0 |
  |   Lines             |           4771 |           4798 |   +27 |
- |   Covered           |           2670 |           2596 |   -74 |
- | Code to Test Ratio  |          1:0.4 |          1:0.4 |  -0.1 |
  |   Code              |          10341 |          10390 |   +49 |
  |   Test              |           4342 |           4342 |     0 |
  | Test Execution Time |          4m29s |          4m29s |    0s |

Code coverage of files in pull request scope (32.9% → 29.7%)

Files	Coverage	+/-
cmd/root.go	0.0%	0.0%
datasource/datasource.go	49.1%	-8.6%

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Reported by octocov