Add call_async_task to thing_server_interface allowing access to BlockingPortal.call

[julianstirling]

In the OpenFlexure Microscope Server cameras make used portal.call() to provide a blocking way to call async code. This provides that same interface by adding thing_server_interface.call_async_task.

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[rwb27]

I should familiarise myself with why this is done in the OFM camera - I thought all the necessary async code was wrapped up on the JPEG streams - but this looks very uncontroversial.

I've suggested a warning that it can deadlock the event loop if it's called there. Should be obvious, but probably worth being explicit...