secret-store: Get rid of old chatty account handling, add sync variant of cm_client_save and cm_utils_get_homeserver (and add it the latter to the public API)
When this code still lived in chatty the labels of secrets were of the form "Chatty password for ".
This changed when chatty switch to using libcmatrix, with 0.7.0~rc0 in October 2022 (and 0.7.0 in February 2023).
It's about time to get rid of this migration layer: Greedily grabbing every secret matching the schema is a recipe for confusion when there can be multiple applications on the same system using libcmatrix.