These changes make the `pulumi.runtime.invoke` function invokable in a
synchronous manner. Because this function still needs to perform
asynchronous work under the covers--namely awaiting a provider URN and
ID if a provider instance is present in the `InvokeOptions`--this
requires some creativity. This creativity comes in the form of a helper
function, `_sync_await`, that performs a logical yield from the
currently running event, manually runs the event loop until the given
future completes, performs a logical resume back to the
currently executing event, and returns the result of the future.
The code in `_sync_await` is a bit scary, as it relies upon knowledge of
(and functions in) the internals of the `asyncio` package. The necessary
work performed in this function was derived from the implementations of
`task_step` (which pointed out the need to call `_{enter,leave}_task`)
and `BaseEventLoop.run_forever` (which illustrated how the event loop is
pumped). In addition to potential breaking changes to these internals,
the code may not work if a user has provided an alternative implementation
for `EventLoop`. That said, the code is a close enough copy of
`BaseEventLoop.run_forever` that it should be a reasonable solution.
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