[data.search.aggs]: Clean up TimeBuckets implementation

[lukeelmers]

The legacy ui/time_buckets was converted to TS & migrated to the aggs service in data.search.aggs, and exists as an implementation detail of the date histogram agg type.

However, there is a lot of cleanup that needs to be done here to remove unused methods & simplify things. This will greatly reduce the maintenance burden of this area of code:

• update config to take explicit ui setting values
  • histogram:barTarget, histogram:maxBars, etc, instead of getConfig function
  • this will make eventual migration to the server easier
• Remove duplicate TimeBuckets interface in the date_histogram agg type and replace with something exported from TimeBuckets
• check to confirm whether __cached__ can be safely removed (I suspect it can as this is not a shared runtime dependency and rather the class is instantiated where it is needed, which is in multiple places)
• Identify any unused methods & remove them if possible
  • these are usually consumed via agg.buckets, etc
• Refactor TimeBuckets class to clean up typings
  • check the arguments provided to setInterval and see if they can be simplified based on usage across Kibana
  • TimeBucketsInterval - check interface to see if all items included are necessary and used elsewhere
• Write unit tests for time_buckets.ts

Parent: #60126

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)