Respect image from task_definition_arn and use default prefect image if no image is provided

[ahuang11]

Replaces #168 with updates from review because I didn't want to manually remove all the image added in the test.

Sets image = Field(default=None) instead of image = Field(default_factory=get_prefect_image_name)

Checks if the input values contain an image or a task definition ARN. If either of those are present, it returns those. If neither are present, it checks the task definition for a Prefect container image and uses that if it is present. If no Prefect container image is found, it uses the default Prefect base image.

Closes #138

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[desertaxle]

LGTM!