feat(core): Add config option for the url preview user agent

conduwuit-example.toml

@@ -1474,6 +1474,12 @@
 #
 #url_preview_check_root_domain = false
 
+# User agent that is used specifically when downloading url previews.
+#
+# example: "continuwuity (bot; +https://forgejo.ellis.link/continuwuation/continuwuity)"
+#
+#url_preview_user_agent = "continuwuity/$VERSION ($VERSION_TAG)"
+
 # List of forbidden room aliases and room IDs as strings of regex
 # patterns.
 #