Migrate ecs-init to aws-sdk-go-v2

Migrate the ecs-init module to aws-sdk-go-v2 because aws-sdk-go will reach end of support on July 31, 2025.

Migrate from session.Session to aws.Config per the docs.
Migrate from ec2metadata.EC2Metadata to imds.Client and update client construction per the docs.
Migrate from s3manager.Downloader to manager.Downloader per the docs.
For the endpoints package,
- Per the docs, "the endpoints package no longer exists in the AWS SDK for Go V2. Each service client now embeds its required AWS endpoint metadata within the client package." However, the client packages only support endpoint resolution (e.g. ECS's EndpointResolver and EndpointResolverV2). The partition and region metadata needed by ecs-init is only available via the internal awsrulesfn package.
- To make the partition and region metadata accessible to ecs-init, use go generate to copy over relevant SDK files [1][2] into an ecs-init package.
- Migrate from endpoints.PartitionForRegion to awsrulesfn.GetPartitionForRegion, which is our own implementation of getPartition. We do not use getPartition because it defaults to the aws partition if it cannot find a match for the region. https://github.com/aws/amazon-ecs-agent/blob/cf8c7a6b65043c550533f330b10aef6d0a342214/ecs-init/config/common.go#L141-L143
- Hardcode in partitions and regions that used to be exported as constants.
Add aws-sdk-go-v2 modules to go.mod in ecs-init.
go mod tidy && go mod verify && go mod vendor
make gogenerate-init

GitHub status checks pass.
Downloaded the ecs-init RPM/DEB packages built from this PR. Installed the packages on EC2 instances running AL2, AL2023, and Ubuntu. Verified that Agent started/stopped/restarted when the systemd ecs-init service was started/stopped/restarted.

New tests cover the changes: no

Migrate ecs-init to aws-sdk-go-v2.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

aws / amazon-ecs-agent