feat: Add support for in-cluster auth

[stepanstipl]

This PR mainly adds support for in-cluster authentication and some minor refactoring on tests.

This func. would deserve integration test, created #236.

[stepanstipl]

Btw. tested this works:

$ kubent-linux-amd64
2:23PM INF >>> Kube No Trouble `kubent` <<<
2:23PM INF version dev (git sha dev)
2:23PM INF Initializing collectors and retrieving data
2:23PM INF Target K8s version is 1.21.1
2:23PM INF Retrieved 0 resources from collector name=Cluster
2:23PM ERR Failed to retrieve data from collector error="secrets is forbidden: User \"system:serviceaccount:default:default\" cannot list resource \"secrets\" in API group \"\" at the cluster scope" name="Helm v2"
2:23PM ERR Failed to retrieve data from collector error="list: failed to list: secrets is forbidden: User \"system:serviceaccount:default:default\" cannot list resource \"secrets\" in API group \"\" at the cluster scope" name="Helm v3"
2:23PM INF Loaded ruleset name=custom.rego.tmpl
2:23PM INF Loaded ruleset name=deprecated-1-16.rego
2:23PM INF Loaded ruleset name=deprecated-1-22.rego
2:23PM INF Loaded ruleset name=deprecated-1-25.rego

(the errors are expected due to limited permissions of default account that got picked up)

And once more planning to ignore Sonar :).