Workload Identity: SPIFFE Federation and Join Method RFD (#43348)

* Start drafting out SPIFFE federation RFD

* Add protos for join flow

* Add more detail

* Remark on guarding the `status` fields from user intervention

* Reorganise join flow description

* Update rfd/0175-spiffe-federation.md

Co-authored-by: Zac Bergquist <[email protected]>

* Add short expalantion of federation

* Default to 5 minutes rather than 60

* Fix copy paste error

---------

Co-authored-by: Zac Bergquist <[email protected]>