Add sso_default_provider_id option and defaulting behavior.

Signed-off-by: Jason Volk <jason@zemos.net>

tuwunel-example.toml

@@ -1834,6 +1834,19 @@
 #
 #one_time_key_limit = 256
 
+# This option is relevant when more than one identity_provider has been
+# configured and `sso_custom_providers_page` is false. Set this option to
+# the `client_id` of the provider to use for requests to
+# `/_matrix/client/v3/login/sso/redirect` (the url lacks a `client_id`).
+#
+# This *must* be configured for some clients (e.g. fluffychat) to work
+# properly when the above conditions require it.
+#
+# When only one identity_provider is configured that will be used as the
+# default and this does not have to be set.
+#
+#sso_default_provider_id = false
+
 # Setting this option to true replaces the list of identity providers on
 # the client's login screen with a single button "Sign in with single
 # sign-on" linking to the URL `/_matrix/client/v3/login/sso/redirect`. The