Allow updating the entire key value store at once

There is an issue I run into with having a typed context.

Say I have a type like:

interface Lead {
    Status: "NONEXISTANT";
} |
{
    Status: "ACTIVE",
    Details: {
    	SomeField: number,
    	SomeOtherField: string
	};
}

And my context looks like:

ctx: restate.ObjectContext<Lead>

Currently there is no typesafe way for me to update the key value store. This is annoying when trying to implement a state-machine pattern for the key value store.

Example:

ctx.set("Status", "ACTIVE"); // Context is now in non-typesafe state because Details doesn't exist
ctx.set("Details", ...) // Get a type error because Details field isn't present when Status is "NONEXISTANT"

If we were to introduce some method to update the entire key/value state at once, we can resolve this. It would be akin to a map operation.

await ctx.update((oldState) => {
	return {
		Status: "ACTIVE",
		Details: {
			SomeField: 1,
			SomeOtherField: "hello"
		}
	}
} // All fields are updated atomically and type safety is preserved

Start with the TypeScript ObjectContext API and the existing ctx.set operation; no file or test is named in the issue. Define the requested whole-state update behavior, including the callback's typed oldState and atomic replacement, then add coverage showing that discriminated-union state transitions remain type-safe.