[Swift] Allow a WorkflowHost to be configured to run on a specific queue

[davidapgar]

As of right now, all workflows assume they are running on the main queue (and additional assertions are being added to ensure this).

While adding assertions, it's been brought up that we should make this configurable:
square/workflow#362 (comment)

Add support for configuring the queue a workflow runs on.

[zach-klippenstein]

FYI we do this in Kotlin via the baseContext parameter to WorkflowHost.Factory (or any of the run methods, which all take a context override), which lets you specify the CoroutineDispatcher to execute the runtime one.

https://github.com/square/workflow/blob/82ecbbc446c07663aa68af31de0b9ad52d1a0878/kotlin/workflow-runtime/src/main/java/com/squareup/workflow/WorkflowHost.kt#L67

https://github.com/square/workflow/blob/82ecbbc446c07663aa68af31de0b9ad52d1a0878/kotlin/workflow-runtime/src/main/java/com/squareup/workflow/WorkflowHost.kt#L84-L88