add constant-expression string utils

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

src/admin/Cargo.toml

@@ -30,6 +30,7 @@ clap.workspace = true
 conduit-api.workspace = true
 conduit-core.workspace = true
 conduit-service.workspace = true
+const-str.workspace = true
 futures-util.workspace = true
 log.workspace = true
 ruma.workspace = true