feat: add a new client-metrics-web package

This adds a lightweight wrapper around the AWS CloudWatch RUM client,
ensuring that we're consistent and can send metrics from other libraries
that we maintain.

This has been through a long design process, mostly outlined here:
https://financialtimes.atlassian.net/wiki/x/aoCJCQI

Release-as: 0.1.0

Author: rowanmanning

.release-please-manifest.json

@@ -1,5 +1,6 @@
 {
   "packages/app-info": "4.1.0",
+  "packages/client-metrics-web": "0.0.0",
   "packages/crash-handler": "5.0.2",
   "packages/errors": "4.0.0",
   "packages/eslint-config": "4.0.0",