Add `wait` parameter to prediction creation methods (#308)

* Add block parameter to prediction creation methods

* Add support for block: or wait: true to run

* Remove unused stream parameter

* Apply suggestions from code review

Co-authored-by: Aron Carroll <aron@replicate.com>

* Replace X-Sync header with Prefer: wait

* Rename block to wait

Update type definitions to capture expectations of wait parameter in run

* Normalize wait value to nonzero integer

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Co-authored-by: Aron Carroll <aron@replicate.com>