Introduce alias for `StoredFields`

[flobernd]

Problem

The Fields type is currently used wherever a collection of fields is expected. In most places, the server accepts the following values:

• missing / null
• [] (no elements)
• ["a"] (single element)
• ["a", "b", ...] (multiple elements)

Unfortunately, there is a special case in all places where fields is used for a stored_fields property, a query parameter etc.. In these cases, special semantics are applied and the following values are accepted:

• missing/null (selects _id and _type [deprecated])
• [] (selects _id and _type [deprecated])
• ["a"] (single element)
• ["a", "b", ...] (multiple elements)
• "_none_" (string; deselects all fields)

While the exact semantics are usually not captured by the specification, this particular case needs to be modeled so that the generators can produce a specific serialization behavior. Elasticsearch does not accept the value _none_ as an element of an array (["_none_"]) but requires it to be encoded as a string. On the other hand, all regular places where fields is used, explicitly do not accept single element as string, but require the use of an array.

Proposal

Introduce a new alias type StoredFields = Fields and use it for all stored_fields properties. Generators can decide whether to just reduce the alias to Fields or to implement special handling.

This mainly affects the statically typed language clients.

cc @swallez @Anaethelion @picandocodigo @l-trotta @stevejgordon

[flobernd]

After giving it a second look, we realized the Fields type in t he specification actually supports Field | Field[]. The .NET generator incorrectly replaced an instance of just Field[] with the Fields type.

No specification fix is required at this point.