WIP: Allow label/target search for ResultStore sources.

[michelle192837]

Instead of requiring label:"prow" in queries, allow any number of label or target atoms in a query. (This still limits the queries we accept, but it should work for most simple uses of ResultStore sources).

Test: Unit tests, and verified with a local instance of TestGrid that a ResultStore source config will pick up results from ResultStore correctly.

[google-oss-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: michelle192837

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[google-oss-prow[bot]]

@michelle192837: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
test-testgrid-all	4d3af7f9a7f400f37e806dcb10288eb40f5cf01c	link	true	/test test-testgrid-all

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[michelle192837]

Found an issue with a growing number of goroutines that didn't pop up on a simple search but does with a more complex search. I need to fix that before this is good to roll out.