feat: add facebook provider (#2007)

* add sign in with facebook

* feat: add facebook provider to factory function

* feat: add facebook config defaults

* feat: use newest facebook api version

* feat: make facebook provider consistent with other providers

* feat: add check for email

We cannot assume a user always has a valid email.
Even though it is not the used "me" endpoint, see:

https://developers.facebook.com/docs/graph-api/reference/user/

* docs: elaborate comment

* fix: fix third party tests

* feat: add facebook icon

* feat: add appsecret_proof to requests w. access token

* refactor: build userinfo url programmatically

* feat: map all available name claims

---------

Co-authored-by: Prathamesh <psvagare@gmail.com>

backend/flow_api/flow/shared/hook_generate_oauth_links.go

@@ -2,9 +2,10 @@ package shared
 
 import (
 	"fmt"
+	"net/url"
+
 	"github.com/labstack/echo/v4"
 	"github.com/teamhanko/hanko/backend/flowpilot"
-	"net/url"
 )
 
 type GenerateOAuthLinks struct {
@@ -38,6 +39,9 @@ func (h GenerateOAuthLinks) Execute(c flowpilot.HookExecutionContext) error {
 	if deps.Cfg.ThirdParty.Providers.Apple.Enabled {
 		c.AddLink(OAuthLink("apple", h.generateHref(deps.HttpContext, "apple", returnToUrl)))
 	}
+	if deps.Cfg.ThirdParty.Providers.Facebook.Enabled {
+		c.AddLink(OAuthLink("facebook", h.generateHref(deps.HttpContext, "facebook", returnToUrl)))
+	}
 
 	return nil
 }