RFC: Session flows

[apascal07]

Doc(s): https://github.com/firebase/genkit/blob/ap/go-session-design/docs/go-session-flow-design.md
PR(s): #4462 (Go)

[MichaelDoyle]

@apascal07 any reason not to merge the rfc's / design docs into the main branch? (or move into a HG issue?). I was going to contribute some diagrams that helped me out.