Add GetUID function to resource object

[malcolmholmes]

The new tests have shown up some naming issues.

We have resource.Name(), which extracts metadata.name, but this is not
the resource's UID, which in some cases needs to be a compound element, e.g.
$NAMESPACE.$NAME for a PrometheusRuleGroup or $TYPE.$JOB for a
SyntheticMonitoringCheck.

This PR adds a GetUID() function that correctly returns a UID across all
resource types.

[dsotirakis]

LGTM, but it would be also nice to add some unit tests, at least for the funcs that have conditions in them :)

[malcolmholmes]

LGTM, but it would be also nice to add some unit tests, at least for the funcs that have conditions in them :)

You got your tests. Had to fix some of the things that were wrong with the code under test, too!