Add a namespace allowlist to loki.source.podlogs

[ptodev]

Fixes grafana/alloy#496. I think the solution should be something along those lines, but I'm not sure what is the best way to test my change?

I don't think I could add unit tests without bootstrapping some sort of k8s environment? In that case, I could either do a one-off local test, or I could try to add something to the integration test suite? I don't think we currently have k8s integration tests.

cc @rfratto, @captncraig, @wildum

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[github-actions]

This PR has not had any activity in the past 30 days, so the needs-attention label has been added to it.
If you do not have enough time to follow up on this PR or you think it's no longer relevant, consider closing it.
The needs-attention label signals to maintainers that something has fallen through the cracks. No action is needed by you; your PR will be kept open and you do not have to respond to this comment. The label will be removed the next time this job runs if there is new activity.
Thank you for your contributions!