new: dev: propose a new generic ``cfg-get-value`` for charm usage.

bin/compose-core

@@ -1586,15 +1586,25 @@ setup_host_resources () {
 }
 export -f setup_host_resources
 
-
-relation-get () {
+## Works on stdin
+cfg-get-value () {
     local key="$1" out
-    if ! out=$(cat "$RELATION_DATA_FILE" | shyaml -y get-value "$key" 2>/dev/null); then
+    if [ -z "$key" ]; then
+        yaml_get_interpret || return 1
+        return 0
+    fi
+    if ! out=$(shyaml -y get-value "$key" 2>/dev/null); then
         err "The key $WHITE$key$NORMAL was not found in relation's data."
         return 1
     fi
     echo "$out" | yaml_get_interpret
 }
+export -f cfg-get-value
+
+
+relation-get () {
+    cfg-get-value "$1" < "$RELATION_DATA_FILE"
+}
 export -f relation-get