Use 'alias' as kube-context username when arg is passed

[sarmad-abualkaz]

Issue #, if available: #4079

Description of changes:
We have a similar issue to #4079, whereby admin users want to perform tests on eks clusters with both developer-like and admin permissions. We use alias's as our way to distinguish between contexts.

With current setup, the following two commands executed one after the other would overwrite the user entry since its sharing the same user-name (i.e. the cluster ARN is always the user-name):
aws eks update-kubeconfig --name mycluster --role-arn arn:aws:iam::123456789123:role/k8s-admin --alias mycluster-admin
aws eks update-kubeconfig --name mycluster --role-arn arn:aws:iam::123456789123:role/k8s-dev --alias mycluster-dev

I feel like an easy win would be to use the --alias name as the user-name for the context, since the alias is anyway being used as the kube-context name. With this change if the alias arg isn't passed, the user-name would continue to be named as before, i.e. cluster-ARN.

@swetashre, since the issue is assigned to you, I was wondering what you think of this approach?

Thanks.

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[codecov-commenter]

Codecov Report

Merging #5413 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5413   +/-   ##
========================================
  Coverage    94.06%   94.06%           
========================================
  Files          190      190           
  Lines        14757    14758    +1     
========================================
+ Hits         13881    13882    +1     
  Misses         876      876           

Impacted Files	Coverage Δ
awscli/customizations/eks/update_kubeconfig.py	97.95% <100.00%> (+0.02%)	⬆️

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[sarmad-abualkaz]

I'm just curious, is anyone actually looking at this PR?

The PR has been open for more than 2 years and I have not received any feedback. If I'm being honest, I've never had such a bad 'open-source' collab experience.

If this is not useful or no longer required, let me know and I can close it. If you have some feedback, also let me know and I can improve on the changes.

Thanks

[splichy]

@kdaily @nateprewitt can you look at this please?

[yuxiang-zhang]

Kindly note that this feature has been added with the --user-alias flag, available since v1.27.98 and v2.11.6.