Allow custom AWS region overrides #1707

[jottofar]

Replicated changes from kubernetes "Add AWS Custom Endpoint capability #70588" into cluster-autoscaler:
Modified k8s cloudprovider aws like kubernetes but exposed key type/methods for use by cluster-autoscaler aws_manager.

Closes #1707

Which issue(s) this PR fixes:

Fixes #1707

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[mwielgus]

Please sign CLA and update the PR title to correctly reflect the content.

[cuppett]

Also, included the text, Closes #1707

[aleksandra-malinowska]

Please don't introduce commented out imports and code unless there's a very good reason for it (state it in comment if this is the case).