[6.x] Improve Bard/Replicator performance during page load

[duncanmcclean]

This pull request aims to improve the performance of Bard & Replicator fields during page load by reducing the payload we generate and send to the client.

Solution

Currently, when we preprocess Bard & Replicator fields, we return new and defaults arrays, which carry the default values and metadata for new sets.

By nature, we only ever need these values when you add a new set. If all you're doing is editing an existing entry, we're generating a bunch of data you don't actually need.

My "solution" is simple:

• Stop returning new and defaults from the fieldtype's metadata
• Perform an AJAX request when you add a set to load it's default values and metadata
  • The values/meta will be cached in a Vue property in case you want to add the same set again, avoiding unnecessary requests.

Benchmarks

I've tested this approach on a real site we've been given access to via support. I've measured the page load of the publish form page.

Before: 3.71s (1235kB)
After: 542ms (121kB)

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Related: #11993

[duncanmcclean]

Passing -1 here might be a little confusing, but according to this PR, it's the intended value when generating the new/defaults arrays.