Add an async API returning CompletableFuture

[garretpremo]

Requested by @DevChango in #1 if the client could offer an async API alongside the synchronous API.

e.g. systemOneAsync returning CompletableFuture.

Proposal

Add async counterparts to the existing entry points:

• CompletableFuture<TypeSafeResponse> systemOneAsync(...) for every systemOne overload
• CompletableFuture<List<ModelCard>> models().listAsync(...)

Both methods should honor the same per-call RequestOptions and the same retry policy as the blocking calls, and complete exceptionally with the same TypeSafeException subclasses.

[garretpremo]

Shipped in 0.3.0: systemOneAsync for every systemOne overload and models().listAsync(), returning CompletableFutures that honor the same per-call RequestOptions and retry policy and complete exceptionally with the same TypeSafeException subclasses as the blocking calls (#5). Artifacts are on their way to Maven Central under io.github.premo-cloud:typesafe-sdk:0.3.0.