Add missing attach method

[akkie]

The other SDKs all have an attach method as described here: await all

The rust SDK misses this method.

I have read in discord that this is planned in a future SDK version, but I have not found an issue.

https://discord.com/channels/1128210118216007792/1364496966142394378

[slinkydeveloper]

Indeed we should do that. The major challenge there is to figure out how to fit that in the response generic type in the InvocationHandle trait, in such a way that is not too painful to use :) The rest of the business logic comes from the sdk-shared-core anyway, and it's easy to wire up (it can be done just like all the other entries)

I'll open this for contributions, in case anyone is interested.