This adds support for synthetic _source to the `version` field type. It
works very similarly to `keyword` but with an extra decode step.
I modified the decoder to return a `BytesRef` instead of a `String`
because many of the callers seemed to be converting that string directly
into bytes again. Synthetic source would have wanted to do that. As was
the query infrastructure.
Now that we're releasing synthetic _source as a tech preview feature, we
no longer want to remove the docs from the non-release builds. And we
want to mark all of the headings describing synthetic `_source` as a
preview.
Currently we have two parameters that control how the source of a document
is stored, `enabled` and `synthetic`, both booleans. However, there are only
three possible combinations of these, with `enabled:false` and `synthetic:true`
being disallowed. To make this easier to reason about, this commit replaces
the `enabled` parameter with a new `mode` parameter, which can take the values
`stored`, `synthetic` and `disabled`. The `mode` parameter cannot be set
in combination with `enabled`, and we will subsequently move towards
deprecating `enabled` entirely.
