add tokenSource support for auth configuration (#4865)

conf/frps_full_example.toml

@@ -105,6 +105,11 @@ auth.method = "token"
 # auth token
 auth.token = "12345678"
 
+# alternatively, you can use tokenSource to load the token from a file
+# this is mutually exclusive with auth.token
+# auth.tokenSource.type = "file"
+# auth.tokenSource.file.path = "/etc/frp/token"
+
 # oidc issuer specifies the issuer to verify OIDC tokens with.
 auth.oidc.issuer = ""
 # oidc audience specifies the audience OIDC tokens should contain when validated.