Use hardcoded partitions and STS hostnames

[gargipanatula]

What this PR does / why we need it:
This PR removes calls to the github.com/aws/aws-sdk-go/aws/endpoints package by porting over the hardcoded data behind the package into a file within the repository. This is meant to move the repository away from the AWS SDK Go V1, which is set to be deprecated on July 31, 2025.

Testing:
Tested the verify command to make sure that it continues to authenticate tokens from within a partition, and rejects tokens from across partitions.

dev-dsk % /aws-iam-authenticator/_output/bin/aws-iam-authenticator verify --cluster-id=panatula627209 --token=$TOKEN
&{ARN:arn:aws:sts::<id>:assumed-role/<role>/<session> CanonicalARN:arn:aws:iam::<id>:role/<role> AccountID:<id> UserID:<userid> SessionName:<session> AccessKeyID:<key> STSEndpoint:sts.us-west-2.amazonaws.com}

dev-dsk % /aws-iam-authenticator/_output/bin/aws-iam-authenticator verify --cluster-id=panatula627209 --token=$TOKEN --partition=aws-cn
could not verify token: input token was not properly formatted: unexpected hostname "sts.us-west-2.amazonaws.com" in pre-signed URL

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: gargipanatula
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[k8s-ci-robot]

Hi @gargipanatula. Thanks for your PR.

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[kmala]

/ok-to-test

[yue9944882]

i think we shouldn't maintain these hardcoded variables in our repo. because it's hard to automate and refresh the list. how about we instead use the endpoint resolver construct from sdk v2 here?

https://github.com/aws/aws-sdk-go-v2/blob/main/service/sts/endpoints.go#L333