Release a Kotlin focused library that supports multiplatform

[kpavlov]

Using Jackson as serialization library has a limits as Jackson is only supported on JVM.
Migrating to kotlinx.serialization would open the possiblilities to make the library multiplatform.

[TomerAberbach]

This library is actually meant to be for the JVM. It's designed specifically for use from Java, which is why we expose Optional instead of nullable types, CompletableFuture instead of suspend functions, etc.

However, we are considering releasing a separate anthropic-kotlin library, which would be meant to be used from Kotlin, and which would support multiplatform