rpadmin: add singleKeyConfig()

`rpk cluster config get` requests only a single cluster property from
`redpanda`.

However, currently we return the entire JSON of the cluster properties
to `rpk`, and then parse the desired property from there.

To support retrieving only a single value from the cluster properties
(and also to support aliased properties in the future), add
`singleKeyConfig()`, which uses a string to be used as the query parameter
`key` to the `redpanda` admin endpoint.

rpadmin/api_config.go

@@ -42,6 +42,19 @@ func (a *AdminAPI) Config(ctx context.Context, includeDefaults bool) (Config, er
 	return unmarshaled, nil
 }
 
+func (a *AdminAPI) SingleKeyConfig(ctx context.Context, key string) (Config, error) {
+	var rawResp []byte
+	err := a.sendAny(ctx, http.MethodGet, fmt.Sprintf("/v1/cluster_config?key=%s", key), nil, &rawResp)
+	if err != nil {
+		return nil, err
+	}
+	var unmarshaled Config
+	if err := json.Unmarshal(rawResp, &unmarshaled); err != nil {
+		return nil, fmt.Errorf("unable to decode response body: %w", err)
+	}
+	return unmarshaled, nil
+}
+
 // SetLogLevel sets the logger level for the logger `name` to the given level
 // for the single admin host in this client. This function will return an error
 // if the client has multiple URLs configured.