Feature/service metadata fixes 2

[aalexandru]

Add a check for available GVKs using a discovery client. If a configured GVK isn't installed on the cluster it will skip it instead of returning an error.

Improve field parsing for servicemetadatawatcher.spec.watchedServiceObjects.watchedFields.src:

• remove "last" as a possible index value, replaced with support for negative indexes (e.g., -1 is the last, -2 second to last etc.)
• add support for maps (e.g., metadata.annotations[some.key/example])
• refactored parsing logic to (hopefully) make it easier to understand