Add null value and Setting type

[swallez]

This PR adds a NullValue type that is an alias to null, meant to represent "meaningful" null values, i.e. nulls that are not equivalent to missing value.

It also introduces a Settings type that represent a setting. Elasticsearch handles settings in very specific ways:

• setting their value to null resets the setting to its default value
• because of how they're implementing on the server, setting values are always returned as strings.

This PR does not uses these new types and value in the API specification. They first need to be implemented in code generators where relevant (a separate issue will track this).

Fixes #2049 - from the proposals in this issue, we introduce a specific type for meaningful nulls, but keep the standard typescript null for "null as optional".

[flobernd]

LGTM!

Maybe we should as well an a behavior that marks a complete class as "Settings". The IndexSettings is a pretty huge type for example and we would need to add the new marker interface to each property.

[swallez]

@flobernd good point the Settings behavior. I hesitated to add it to this PR, but finally decided to keep it small so that we can unlock progress on #2367.

We'll add it in a separate PR, and it will indeed avoid heavy modifications of the various setting types. However, I consider this Settings behavior as something that should be processed during the compilation phase by updating every property of a scalar type to be a Setting<T> so that we don't have to implement that in every code generator. So a bit of additional work that also mandates a separate PR. WDYT of this approach?