Follow-up from "Add Knative metrics to Prometheus" - Use Vuex for Vue object store

The following discussion from !24663 (merged) should be addressed:

• @wortschi started a discussion: (+2 comments)

  Question:

  @cab105 I know that this wasn't introduced in this MR but I noticed that we're using some kind of custom store implementation here. So I'm wondering if it would make sense to use Vuex instead? Things might get a bit messy once we need to add more data to the store. Might be worth to double check with a maintainer to see if we should tackle this in a separate issue. /cc @iamphill @filipa

The response from @filipa is yes. Given that there are two session stores in use: function_details to handle the graphData and per-function components, and functions to handle the collection of functions mapped to their cluster's environment scope, this issue is to investigate and possibly bring both session stores into a single Vuex session store.