[ECS:Plugin]: Supporting controller type "ECS"

[armistcxy]

What this PR does: This PR adds support for the native ECS deployment controller type by introducing a strategy pattern:

• provider layer changes:
  • ForceNewDeployment method added to the ECS interface and client struct, it calls UpdateService with ForceNewDeployment: true and forwards DeploymentConfiguration (with the support of AWS SDK v1.78.0 we can specify Strategy: ROLLING, BLUE_GREEN, CANARY, LINEAR so AWS-managed deployment strategies are supported).
  • CreateService and UpdateService now branch on controller type:
    • For EXTERNAL the existing task-set-based path is preserved
    • For ECS controller, additional fields (TaskDefinition, LaunchType, NetworkConfiguration, ServiceRegistries, DeploymentConfiguration) are included in the API call.
• deployment layer changes:
  • applyServiceDefinition now returns a newlyCreated bool so callers can distinguish create vs. update.
  • New controller.go defines the deploymentController interface with Sync, PrimaryRollout, and Rollback methods, a newDeploymentController(serviceDef) factory, and two concrete implementations:
    • externalController: simply move of the existing task-set-based flow
    • ecsController: register task definition -> inject task def ARN into service -> applyServiceDefinition (create or update) -> call ForceNewDeployment only for existing services (newly created services start their first deployment automatically) -> WaitServiceStable.

Stage comparison

Stage	EXTERNAL controller	ECS controller
ECS_SYNC	task-set flow (unchanged)	ForceNewDeployment flow
ECS_PRIMARY_ROLLOUT	task-set flow (unchanged)	ForceNewDeployment flow
ECS_ROLLBACK	task-set flow (unchanged)	ForceNewDeployment flow
ECS_CANARY_ROLLOUT	supported	error
ECS_CANARY_CLEAN	supported	no-op success
ECS_TRAFFIC_ROUTING	supported	error

Why we need it: Previously the ECS plugin hard-coded the EXTERNAL deployment controller, this blocks native ECS features such as Service Connect, deployment circuit breakers, and deployment alarms that are only available with the ECS controller type.

Supported features wih ECS controller

Feature	Brief Description
Service Connect	Native service-to-service discovery and communication mesh managed by ECS; not available with EXTERNAL controller
Deployment Circuit Breaker	Automatically detects a failing deployment and triggers a rollback without manual intervention
Deployment Alarms	Integrates CloudWatch alarms into the deployment lifecycle — halts or rolls back if an alarm fires during rollout
Rolling Update Strategy	AWS gradually replaces old tasks with new ones, controlling minimumHealthyPercent / maximumPercent natively
Blue/Green Strategy	Runs the new version alongside the old; cuts over atomically after a configurable bake time
Canary Strategy	Shifts a small percentage (CanaryPercent) of traffic to the new version first, then completes the rollout after CanaryBakeTimeInMinutes
Linear Strategy	Incrementally increases traffic to the new version in equal steps (StepPercent) at fixed intervals (StepBakeTimeInMinutes)
Bake Time	Configurable wait period after a deployment step before AWS declares the deployment stable and proceeds
Application Auto Scaling	Auto Scaling adjusts desiredCount in sync with the native ECS deployment lifecycle, avoiding race conditions that arise when scaling interacts with the task-set management of the EXTERNAL controller
Service Registries (Cloud Map)	Registers the service with AWS Cloud Map for DNS-based service discovery; properly passed to CreateService / UpdateService for the ECS controller
VPC Lattice Integration	Connects the ECS service to a VPC Lattice target group for cross-VPC / cross-account load balancing; requires ECS controller type
Managed Instance Draining	Automatically drains tasks from EC2 instances being terminated (scale-in events) without manual intervention; only supported with ECS controller

All deployment strategies (Rolling, Blue/Green, Canary, Linear) are configured via DeploymentConfiguration.Strategy in the service definition (available since aws-sdk-go-v2/service/ecs v1.78.0).

Which issue(s) this PR fixes: Part of #6443