connector: implement LinkedIn connector

connector/linkedin implements authorization strategy via LinkedIn's
OAuth2 endpoint + profile API.

It doesn't implement RefreshConnector as LinkedIn doesn't provide any
refresh token at all (https://developer.linkedin.com/docs/oauth2, Step 5
— Refresh your Access Tokens) and recommends ordinary AuthCode exchange
flow when token refresh is required.

Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>

server/server.go

@@ -24,6 +24,7 @@ import (
 	"github.com/coreos/dex/connector/github"
 	"github.com/coreos/dex/connector/gitlab"
 	"github.com/coreos/dex/connector/ldap"
+	"github.com/coreos/dex/connector/linkedin"
 	"github.com/coreos/dex/connector/mock"
 	"github.com/coreos/dex/connector/oidc"
 	"github.com/coreos/dex/connector/saml"
@@ -409,6 +410,7 @@ var ConnectorsConfig = map[string]func() ConnectorConfig{
 	"oidc":         func() ConnectorConfig { return new(oidc.Config) },
 	"saml":         func() ConnectorConfig { return new(saml.Config) },
 	"authproxy":    func() ConnectorConfig { return new(authproxy.Config) },
+	"linkedin":     func() ConnectorConfig { return new(linkedin.Config) },
 	// Keep around for backwards compatibility.
 	"samlExperimental": func() ConnectorConfig { return new(saml.Config) },
 }