Add support for Smithy IDLv 2.0 default trait

[ianbotsf]

Add support for the defaultTrait.

• In particular we may want to update our codegen constructs which are currently based on things like @boxed and primitive value types for figuring out defaults and nullability. We should revamp this to be much clearer and to align to IDL2 (example). This will require using the nullability index (possibly in more places than we do now).
• @box trait was deprecated, update tests/codegen to use default trait instead
• Implement presence tracking

[ianbotsf]

Support Smithy IDL v2, including int enums, full nullability support via presence tracking, and any other new features available (requires investigation).

[aajtodd]

Presence tracking: https://smithy.io/2.0/spec/type-refinement-traits.html#default-value-serialization

[aajtodd]

Reviewing the changelog for Smithy we need look into:

• Set shape was deprecated. Done in Fix smithy sets #673
• Migrate to enum shapes. Done in 8f3279a
• Add support for IntEnum #752
• Migrate to DirectedCodegen #753

[aajtodd]

I've split this catch all Smithy IDL v2 issue out into multiple issues (noted in comment above). I've left this issue dedicated to supporting the @default trait in Smithy IDL v2

[aajtodd]

UPDATE: Guidance has recently changed and presence tracking is no longer the recommendation

smithy-lang/smithy@609cb16