Add Smithy IDL support

[clemensv]

Feature request

Add conversion support for Smithy IDL.

Motivation

Smithy is common for service/data modeling (especially AWS ecosystems); schema conversion would improve interoperability.

Suggested scope

• Import: smithy2a (Smithy -> Avrotize Schema)
• Export: ^G2smithy (Avrotize Schema -> Smithy model subset)
• Handle structures, unions, enums, lists/maps, traits relevant to data shape
• Keep service/operation modeling explicitly scoped (in/out of phase 1)

Acceptance criteria

• Commands + docs
• Tests for core data-shape constructs and references
• Explicit behavior for service-level constructs

Additional requirement: JSON Structure conversions

Please also include JSON Structure conversion support:

• smithy2s (Smithy -> JSON Structure)
• s2smithy (JSON Structure -> Smithy data-shape subset)
• Keep service/operation modeling scope explicit.

[clemensv]

Additional requirement for this issue:
Please include JSON Structure conversion support in scope as well (import to JSON Structure and export from JSON Structure), with tests and documented mapping limitations where needed.