Add executor.imagePullPolicy to task creation payload

Suggestion:

In the task creation endpoint payload, add to the `executor` definition an optional `imagePullPolicy` field which can be set to one of ["Always", "IfNotPresent", "Never"]. The default value can be defined by the server or specified in the spec.

```
{
"name": "string",
"executors": [
{
"image": "ubuntu:20.04",
"imagePullPolicy": "",
[...]
}
]
[...]
}
```

As a workaround, my server is currently accepting it as a task tag, which forces us to apply it to either _all_ or _none_ of a task's executors.

Start at the task creation endpoint schema and inspect the executor definition, since no repository file or test is named. Add the optional imagePullPolicy field with the three permitted values and an agreed default, then confirm that task creation payload validation accepts it.

Written by the indexing model from the issue text.