Support K8s custom resources

[tortila]

Add support for custom resources.

Closes #22

Description

Rationale for this feature can be found in the corresponding issue: #22.

Some decisions that I made along the way:

• Manifest's property kind is of type ResourceType, which is an Enum. I decided not to extend this Enum and not to change the type of this property. Instead, the custom manifests that are found will have kind of equal to ResourceType.OTHER, which has a meaningless value of .... In order to get the real kind of the resource as defined in the custom manifest, its property .body["kind"] needs to be looked up instead.
• I decided to only support resources which are scoped to a namespace. This limitation allows us not to worry about the cleanup, as the deployed custom resource will be deleted along with the namespace (unless someone decides to deploy this resource to a different namespace... which is always possible even for the default and required resources!). This means that some custom resources (e.g. with scope equal to Cluster) will be ignored.

Additionally, as mentioned in the issue: fetching of the available custom resources in the cluster is not a convenience feature, but rather a prerequisite. Call to create a custom resource demands a plural which can only be found on the custom resource definition (and not in the manifest).

Types of Changes

• New feature (non-breaking change which adds functionality)
• Documentation

Review

Reviewers' checklist:

• If this PR implements new flows or changes existing ones, are there good tests for these flows? If this PR rather removes flows, are the obsolete tests removed as well?
• Is the documentation still up-to-date and exhaustive? This covers both technical (in source files) and functional (under docs/) documentation.
• Is the changelog updated?
• Does the new version number correspond to the actual changes from this PR? In doubt, refer to https://semver.org.

[zincr[bot]]

🤖 zincr found 0 problems , 1 warning

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• ℹ️ tests/kubernetes/test_client.py had +100 lines of code changed in a single commit Please review this file to determine the source of this change  

[tortila]

I will merge with the Codacy warning, as it concerns pytest specific functions and is only for tests.