feat: send kube version in user-agent of aiven API clients

[jeff-held-aiven]

No description provided.

[rriski]

LGTM, although now we have two places where operatorVersion is specified. It would be good to have a single source of truth.

https://github.com/aiven/aiven-operator/pull/655/files#diff-96e66e1ccb0083ac5a2b7fc37cb002803d242ea67c1981a5d09ef8ee2668354cR38-R40 + https://github.com/aiven/aiven-operator/pull/655/files#diff-2873f79a86c0d8b3335cd7731b0ecf7dd4301eb19a82ef7a1cba7589b5252261R32-R34

[Serpentiel]

As @rriski mentioned, we need to get rid of the additional definition of the provider version.

We also should make the approach similar to the one in our Terraform Provider, here's how it works:

• version is defined somewhere, preferably in main.go.
• It is then overwritten during the compile time via go build's ldflags parameter, similarly to this approach.

TL;DR. While I think it's unreasonable to add GoReleaser support in this PR, I think it'll be beneficial for us to define the meaningful operatorVersion in one place only; this could be done by setting the one in suite_test.go to always be a constant string test, like we do over here. This also allows to understand, via the User-Agent, which requests originated from our tests, and which haven't.

[Serpentiel]

lgtm