Agent identity establishment and verification (SPIFFE/SPIRE) Playbook

[husky-parul]

Create a playbook for agent identity and verification using SPIFFE

Related to #52

[sarahnovotny]

Thanks @husky-parul. Adding the playbook label and linking back to the parent Identity and Authorization Playbook in #52.

Per your commitment at the WS4 Apr 23 meeting, tracking this as a partial deliverable feeding into #52. Will confirm scope and cadence at the Apr 30 WS4 meeting.