Adds permissions for ECS agent to run ECSTasks

[EmilRex]

Description

This PR changes the default behavior of the agent_task_role_arn variable. If the variable is set, everything behaves as before, in that the provided role is set as the Prefect agent service's task role. However, if the variable is not set, then we will now create a default task role that includes all permissions necessary for the agent to be able to use the ECSTask infrastructure type. Specifically,

ec2:DescribeSubnets
ec2:DescribeVpcs
ecr:BatchCheckLayerAvailability
ecr:BatchGetImage
ecr:GetAuthorizationToken
ecr:GetDownloadUrlForLayer
ecs:DeregisterTaskDefinition
ecs:DescribeTasks
ecs:RegisterTaskDefinition
ecs:RunTask
iam:PassRole
logs:CreateLogGroup
logs:CreateLogStream
logs:GetLogEvents
logs:PutLogEvents

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

New Recipe Checklist

• [x] My PR is in the format of Add <project-name> recipe
• [x] My recipe is reproducible and explains everything needed to run successfully. If my code has external dependencies, I make mention of them.
• [x] My code is easily understandable and/or well-commented.
• [x] If my recipe requires a new category (e.g. creating a monitoring/ folder in devops/), I have encapsulated my project within its own subfolder so that others can add their own recipes as well.
• [x] If my recipe uses Prefect < 2.0, my code is within the prefect-v1-legacy/ folder.