Automatic merge from submit-queue
Move private key parsing from serviceaccount/jwt.go to client-go/util/cert
**What this PR does / why we need it**:
Unify private key parsing from serviceaccount/jwt.go into the client-go library.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*:
Partial fix to #40807 - only private key functions.
**Special notes for your reviewer**:
**Release note**:
```release-note
Move private key parsing from serviceaccount/jwt.go to client-go/util/cert
```
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