Automatic merge from submit-queue (batch tested with PRs 50119, 48366, 47181, 41611, 49547)

Move remaining cert helper functions from pkg/serviceaccount to client-go/util/cert

**What this PR does / why we need it**:
Unifies all remaining certificate helper functions from pkh/serviceaccount to client-go/util/cert. Private key functions were moved in #40907

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #40807

**Special notes for your reviewer**:

**Release note**: