Automatic merge from submit-queue (batch tested with PRs 43558, 48261, 42376, 46803, 47058)

OpenAPI downloads protobuf rather than Json

**What this PR does / why we need it**: 
The current implementation of the OpenAPI getter fetches the swagger in a Json format from the apiserver. The Json file is big (~1.7mb), which means that it takes a long time to download, and then a long time to parse. Because that is going to be needed on each `kubectl` run later, we want this to be as fast as possible.

The apiserver has been modified to be able to return a protobuf version of the swagger, which this patch intends to use.

Note that there is currently no piece of code that exists that allows us to go from the protobuf version of the file, back into Json and/or `spec.Swagger`. Because the protobuf is not very different (but significantly different enough that it can't be translated), I've updated the code to use `openapi_v2.Document` (the protobuf type) everywhere rather than `spec.Swagger`. The behavior should be identical though.

There are more changes that are coming in follow-up pull-requests: using the gzip version (also provided by the new apiserver) to even further reduce the size of the downloaded content, and use the HTTP Etag cache mechanism to completely get rid of recurrent fetch requests. I'm currently working on these two features.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: partly #38637

**Special notes for your reviewer**:

**Release note**:
```release-note
NONE
```