add constant-expression string utils

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

src/router/Cargo.toml

@@ -55,6 +55,7 @@ conduit-admin.workspace = true
 conduit-api.workspace = true
 conduit-core.workspace = true
 conduit-service.workspace = true
+const-str.workspace = true
 log.workspace = true
 tokio.workspace = true
 tower.workspace = true