text: Title: Added support for external signing of Service Account tokens
  Body: Initial implementation PR for kubernetes/enhancements#704
  
  
  **What type of PR is this?**
  /kind feature
  
  **What this PR does / why we need it**:
  
  Adds support for external signing of Projected ServiceAccount tokens.
  
  **Which issue(s) this PR fixes**:
  
  **Special notes for your reviewer**:
  
  Tests forthcoming
  
  **Does this PR introduce a user-facing change?**:
  
  ```release-note
  Added support for external signing of Service Account tokens
  ```
  