Add some helpers to the sdk for working with namespaces.

[ncabatoff]

These are some tools for working with namespaces that I'm pulling out of a bigger PR. I don't want to put them in the API right now, both because one of them is almost certainly test-only (DeleteAllNamespaces), and because I think there are more basic CRUD namespace operations missing from the API which I don't have time to implement right now. I think they're better in the sdk rather than vault proper because I could imagine external repos wanting to use them.

[github-actions]

CI Results:
Failures:

Test Type	Package	Test	Logs
race	vault	TestOIDC_PeriodicFunc	view test results
race	vault	TestOIDC_PeriodicFunc/test-key	view test results

[github-actions]

Build Results:
All builds succeeded! ✅

[VioletHynes]

These seem like good additions, I'm not averse to these being added, but is there a use case we're supporting here? To me it seems like stuff like namespace.Canonicalize or namespace.SplitIDFromString would be more broadly useful in the SDK. I'm also a little squeamish about these being added without corresponding tests (or tests that consume them).

[ncabatoff]

These seem like good additions, I'm not averse to these being added, but is there a use case we're supporting here? To me it seems like stuff like namespace.Canonicalize or namespace.SplitIDFromString would be more broadly useful in the SDK. I'm also a little squeamish about these being added without corresponding tests (or tests that consume them).

The main use case is when running multiple tests using the same cluster and you want to clean up any namespaces created by the previous test.

The code is exercised in some other PRs I'm working on on the ent side.