Add support for ternary Removable type, to facilitate "merge-patch"-like usecases

[Baccata]

Adds a smithy4s.Removable construct, to help model patchable data where a field can be in three different states :

1. present
2. absent
3. removed

At the schema level, it is modelled as Option[Option[A]] which prevents yet another impactful change. The alloy#nullable trait is also expected to be applied on the field. Without it the deserialisation cannot distinguish between Option[None] and None.

At the codegen level, the signal to render Removable is the presence of the alloy#nullable trait on a field that's not also annotated with smithy.api#required.

TODOs

• In alloy, add the alloy#nullable to the list of traits known by the simpleRestJson protocol
• See whether the addition of compliance tests is doable (it may not be the case, considering how smithy works)
• add documentation in both alloy and smithy4s
• add support in the dynamic module

[daddykotex]

minor we have schema.hints.has which might make this easier to read

[lewisjkl]

In alloy, add the alloy#nullable to the list of traits known by the simpleRestJson protocol

If I'm not mistaken, this means we'll need to do a minor version bump (rather than patch) as to not mess with any existing functionality. I think it probably wouldn't mess anything up, but I'm not positive since it will change the HintMasks we derive from the protocolTraits

[Baccata]

Superseded by #1408