Added agent.ua_string as a new contextual attributes for web base schemas

Added agent.ua_string as a new property for web base schemas to allow us to use it as a new contextual attribute for Experiment Lab client libraries:

• Updated fragment/analytics/product_metrics/common to have agent.ua_string as a new property
• Updated analytics/product_metrics/web/base , analytics/product_metrics/app/base and analytics/product_metrics/web/translation accordingly

Bug: T385180

Merge request checklist

• Phabricator task(s) associated with this change added to commit message (use Bug: TASK_NUMBER on line at end of commit message)
• wikitech:Event Platform/Instrumentation How To#Writing MediaWiki instrumentation code was followed for the schema(s) in the merge request. This includes if this merge request entails a new MEDIUM or HIGH risk schema, or if this merge request entails a schema alteration that increases the risk level based on Data Collection Guidelines.
• [N/A] If this introduces a new or upgraded MEDIUM or HIGH risk level, @sguebo added as a reviewer (please check @sguebo calendar for availability and find an alternate if @sguebo is out).
• wikitech:Event Platform/Schemas/Guidelines was followed for the schema(s) in the merge request.
• [N/A] Links to any other data specifications, such as a measurement plan or instrumentation specification if pertinent. Links to related code / Gerrit/GitLab patch pertinent to this schema change if it exists yet; if it doesn't exist yet that is okay and the merge requester should be sure to ensure reachability for that code via the commit message's referenced Phabricator task(s) later on. Please add links for any data specifications and relevant patches below this checkbox line if you have them.