Allow users to implement update asynchronously in Java natively

Is your feature request related to a problem? Please describe.
Currently update handlers in Java are all run in their own threads. This means that if the update handler takes a long time and multiple updates are sent the workflow execution can consume lots of threads just waiting in update handlers.

Describe the solution you'd like
Support the ability to transform an update call into a Promise that can be completed later in another thread like the main workflow method .

Additional context
We have a similar feature to support async activities https://javadoc.io/doc/io.temporal/temporal-sdk/1.0.6/io/temporal/activity/ActivityExecutionContext.html#isUseLocalManualCompletion--

Currently users can do this with the existing SDK API, but it will still consume a thread waiting for the result to return in the handle. If the SDK had a native API for this we wouldn't need to wait in the thread.

Start by reviewing the existing async activity support referenced through ActivityExecutionContext.isUseLocalManualCompletion and compare it with the current update-handler behavior described in the issue. Done should provide a native way to complete an update later without keeping a thread waiting for the result.