OAuth 2.0 Token Exchange (#2806)

cmd/dex/config.go

cmd/dex/config_test.go

cmd/dex/serve.go

connector/connector.go

connector/mock/connectortest.go

connector/oidc/oidc.go

connector/oidc/oidc_test.go

server/handlers.go

server/handlers_test.go

server/oauth2.go

server/oauth2_test.go

server/refreshhandlers.go

server/server.go

server/server_test.go