Adding get-token implementation for EKS

[sgundapu]

Issue #, if available:

Description of changes:
Adding get-token implementation for EKS

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[codecov-io]

Codecov Report

Merging #4141 into develop will decrease coverage by 0.22%.
The diff coverage is 73.77%.

@@             Coverage Diff             @@
##           develop    #4141      +/-   ##
===========================================
- Coverage    94.46%   94.24%   -0.23%     
===========================================
  Files          187      188       +1     
  Lines        14121    14170      +49     
===========================================
+ Hits         13340    13355      +15     
- Misses         781      815      +34

Impacted Files	Coverage Δ
awscli/customizations/eks/update_kubeconfig.py	97.91% <ø> (-0.24%)	⬇️
awscli/customizations/eks/__init__.py	100% <100%> (ø)	⬆️
awscli/customizations/eks/get_token.py	72.88% <72.88%> (ø)
awscli/compat.py	78.12% <0%> (-9.38%)	⬇️

---

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[codecov-io]

Codecov Report

Merging #4141 into develop will decrease coverage by 0.02%.
The diff coverage is 74.19%.

@@             Coverage Diff             @@
##           develop    #4141      +/-   ##
===========================================
- Coverage    94.46%   94.43%   -0.03%     
===========================================
  Files          187      188       +1     
  Lines        14121    14136      +15     
===========================================
+ Hits         13340    13350      +10     
- Misses         781      786       +5

Impacted Files	Coverage Δ
awscli/customizations/eks/update_kubeconfig.py	97.91% <ø> (-0.24%)	⬇️
awscli/customizations/eks/__init__.py	100% <100%> (ø)	⬆️
awscli/compat.py	91.77% <100%> (+4.27%)	⬆️
awscli/customizations/eks/get_token.py	72.88% <72.88%> (ø)

---

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Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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