Add the new `username` scope to the Supervisor and exclude usernames from dynamic clients which are not granted the scope, and other dynamic client related changes

[cfryanr]

• Create a new username scope, required for clients to get username in ID token
• For backwards compatibility with older Pinniped CLIs, the pinniped-cli client does not need to request the username or groups scopes for them to be granted. For dynamic clients, the usual OAuth2 rules apply: the client must be allowed to request the scopes according to its configuration, and the client must actually request the scopes in the authorization request.
• If the username scope was not granted, then there will be no username in the ID token, and the cluster-scoped token exchange will fail since there would be no username in the resulting cluster-scoped ID token.
• The OIDC well-known discovery endpoint lists the username and groups scopes in the scopes_supported list, and lists the username and groups claims in the claims_supported list.
• Add username and groups scopes to the default list of scopes put into kubeconfig files by "pinniped get kubeconfig" CLI command, and the default list of scopes used by "pinniped login oidc" when no list of scopes is specified in the kubeconfig file
• The warning header about group memberships changing during upstream refresh will only be sent to the pinniped-cli client, since it is only intended for kubectl and it could leak the username to the client (which may not have the username scope granted) through the warning message text.
• Add the user's username to the session storage as a new field, so that during upstream refresh we can compare the original username from the initial authorization to the refreshed username, even in the case when the username scope was not granted (and therefore the username is not stored in the ID token claims of the session storage)
• Bump the Supervisor session storage format version from 2 to 3 due to the username field being added to the session struct
• Extract commonly used string constants related to OIDC flows to api package.
• Change some import names to make them consistent:
  • Always import github.com/coreos/go-oidc/v3/oidc as "coreosoidc"
  • Always import go.pinniped.dev/generated/latest/apis/supervisor/oidc as "oidcapi"
  • Always import go.pinniped.dev/internal/oidc as "oidc"
• pinniped get kubeconfig errors when the token exchange request audience value includes the reserved substring .pinniped.dev to give faster feedback in this corner case, since the token exchange endpoint would reject that request anyways
• Always add the azp claim to ID tokens to show the original client ID

Release note:

TODO

[codecov[bot]]

Codecov Report

Merging #1249 (0bb2c7b) into dynamic_clients (6b29082) will decrease coverage by 0.08%. The diff coverage is 73.33%.

@@                 Coverage Diff                 @@
##           dynamic_clients    #1249      +/-   ##
===================================================
- Coverage            76.72%   76.64%   -0.09%     
===================================================
  Files                  164      164              
  Lines                11211    11236      +25     
===================================================
+ Hits                  8602     8612      +10     
- Misses                2323     2339      +16     
+ Partials               286      285       -1     

Impacted Files	Coverage Δ
...ler/authenticator/jwtcachefiller/jwtcachefiller.go	87.50% <ø> (ø)
...sorconfig/oidcclientwatcher/oidc_client_watcher.go	90.62% <ø> (ø)
internal/fositestorage/accesstoken/accesstoken.go	96.15% <ø> (ø)
...sitestorage/authorizationcode/authorizationcode.go	88.52% <ø> (ø)
...ernal/fositestorage/openidconnect/openidconnect.go	87.50% <ø> (ø)
internal/fositestorage/pkce/pkce.go	94.11% <ø> (ø)
...nternal/fositestorage/refreshtoken/refreshtoken.go	96.29% <ø> (ø)
internal/oidc/oidc.go	0.00% <0.00%> (ø)
internal/oidc/token_exchange.go	0.00% <0.00%> (ø)
internal/testutil/oidcclient.go	0.00% <0.00%> (ø)
... and 17 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.