Add ability to embed and use OAuth creds in a waypoint token

[evanphx]

This opens up the authentication modes for waypoint clients talking to a server to be able to delegate the authentication to an OAuth2 provider. This is only client support, the server does not yet expose this functionality, but it is available for future server implementations to use.

[xiaolin-ninja]

                                  (Not in the PR)                                    
                     ┌───────────8. OAuth dance to ─────────────────┐                
                     │            validate token.                   │                
                     │                                              ▼                
                     │                                        ┌──────────┐           
                     │                                        │  OAuth   │           
                     │                                     ┌──│  Server  │◀─┐        
                     │                                     │  │          │  │        
                     │                                     │  └──────────┘  │        
                     │                                     │                │        
                     │                                     │                │        
                     │                                6. Yeah ok, I         │        
                     │                                  know you.      5. Can I have 
                     │   ┌─────────────────────┐           │             an OAuth    
                     │   │                     │           │            token pls?   
                     ▼   ▼                     │           └────────┐    Here's my   
                  ┌────────────┐             7. Hi! Can             │      info.     
                  │  Waypoint  │             you handle             │       │        
               ┌─▶│   Server   │──────┐        │this                │       │        
               │  │            │      │       request,              │       │        
               │  └────────────┘      │      here's my              ▼       │        
               │                      │        OAuth                Λ       │        
               │                2. Here you    token.              ╱ ╲      │        
               │                   go, I       │                  ╱   ╲     │        
               │                smuggled an    │                 ╱     ╲    │        
               │                   OAuth       │                ╱       ╲   │        
           1. I want              server       └───────────────▕ client  ▏──┘        
           a Waypoint            address &                      ╲       ╱            
             token              credentials                      ╲    4. Oh, there's 
               │                  in it.                          ╲   OAuth info in  
               │                      │                            ╲ ╱    this!      
               │                      │                             V                
               │                      │                             ▲                
               │                      │                             │                
               │            .─────.   │                        3. Client, do         
               │           ; User  :  │                         your thing.          
               └───────────:       ;◀─┘                          Here's my           
                            ╲     ╱                              Waypoint            
                             `───'                             access token          
                               │                                    │                
                               │                                    │                
                               └────────────────────────────────────┘                

[xiaolin-ninja]

Here is the gist version of the OAuth flow diagram above.

[izaaklauer]

Not ideal to duplicate the token magic, but not the worst.

[evanphx]

Going to leave the duplication for now, refactoring for this one thing is more ugly.

[briancain]

Looks good! We went over this in a team zoom and I asked my questions there 😄