[ML] Copy over TimeBuckets into ML code

[timroes]

ML is currently making heavy usage of TimeBuckets (ui/time_buckets). In preparation for the new platform this will become part of the data plugin, but we don't want to expose it as an API, since it's more like an implementation detail of how AggConfigs work. Also we are planning to do some refactoring around it, that might break it's API. Since we're not considering this to be an API in the future, and you want to keep using it, I would like to ask you to copy over the relevant code (basically the ui/public/time_buckets folder) into the ML plugin, so we can in the future just remove access to that code for anything outside the data plugin, and also are not destroying ML code while changing the TimeBuckets functionality.

This is not a super urgent matter, but it would be good if this could happen sometime until around the 7.5 release.

[elasticmachine]

Pinging @elastic/ml-ui

[peteharverson]

@timroes thanks for raising this. We will schedule this for 7.5.