feat: adds the preemptible task hint

CHANGELOG.md

@@ -20,6 +20,8 @@ Keep the changelog pleasant to read in the text editor:
 version 1.3.0
 ---------------------------
 
++ Introduced the `preemptible` task hint ([#728](https://github.com/openwdl/wdl/pull/728)).
+
 + Documents may now load any document with the same major version and a minor version that is less than or equal to that document's version ([#698](https://github.com/openwdl/wdl/pull/698)).
 
 version 1.2.0

SPEC.md

@@ -112,6 +112,7 @@ Revisions to this specification are made periodically in order to correct errors
         - [`localization_optional`](#localization_optional)
         - [`inputs`](#inputs)
         - [`outputs`](#outputs)
+        - [`preemptible`](#-preemptible)
       - [Compute Environments](#compute-environments)
       - [Conventions and Best Practices](#conventions-and-best-practices)
     - [🗑 Runtime Section](#-runtime-section)
@@ -5562,6 +5563,25 @@ Reserved input-specific attributes:
 
 Provides output-specific hints. Each key must refer to a parameter defined in the task's [`output`](#task-outputs) section. A key may also use dotted notation to refer to a specific member of a struct output.
 
+##### ✨ `preemptible`
+
+* Accepted types: `Int`
+* Default value: `0`
+
+A hint to the execution engine that a task _may_ be tried using a preemptible instance the specified number of times. Engines that are not configured to use or do not support preemptible instances may ignore this parameter and the implied number of retries completely.
+
+Engines should only count failed tasks against the `preemptible` count if the reason for the failure was preemption. Other failures, such as unexpected non-zero exit codes, should be counted against the [`max_retries`](#max_retries) requirement instead.
+
+A value of `0` means the task should not be tried with a preemptible instance.
+
+```wdl
+task preemptible_example {
+  hints {
+    preemptible: 3
+  }
+}
+```
+
 #### Compute Environments
 
 The `hints` section should be used to provide hints that are specific to different compute environments such as HPC systems or cloud platforms. Attributes for a compute environment should be specified in a `hints` value, in which any of the [reserved hints](#reserved-task-hints) are allowed to override the values specified at the task level (if any), and other attributes are platform-specific.