Implement SEP-1034: Default Values for Elicitation Schemas

[devcrocod]

This is a tracking issue for implementation of SEP-1034

Summary

This SEP extends the elicitation feature by adding support for default values across all primitive schema types (StringSchema, NumberSchema, and EnumSchema). The Kotlin SDK needs to implement the optional default field for these schema types, ensuring servers can pre-populate elicitation forms with reasonable default values to improve user experience and reduce manual input requirements. The feature must remain backward compatible as the default field is optional.

Currently, the Kotlin SDK uses JsonObject for elicitation schema properties without typed schema classes. This implementation will require creating typed schema classes (StringSchema, NumberSchema, EnumSchema, BooleanSchema) with support for the default field, updating DSL builders for convenient schema creation, and maintaining backward compatibility with the existing JsonObject-based approach.