Support context params

[stealthycoin]

Based off of boto/botocore#2740

[codecov-commenter]

Codecov Report

Merging #7423 (babab33) into endpoints-2.0 (8e3fcb2) will increase coverage by 0.02%.
The diff coverage is 97.92%.

❗ Current head babab33 differs from pull request most recent head 766bb0d. Consider uploading reports for the commit 766bb0d to get more accurate results

@@                Coverage Diff                @@
##           endpoints-2.0    #7423      +/-   ##
=================================================
+ Coverage          93.71%   93.74%   +0.02%     
=================================================
  Files                351      352       +1     
  Lines              36175    36510     +335     
  Branches            5202     5265      +63     
=================================================
+ Hits               33903    34227     +324     
- Misses              1648     1655       +7     
- Partials             624      628       +4     

Impacted Files	Coverage Δ
awscli/botocore/model.py	97.44% <91.66%> (-0.35%)	⬇️
awscli/botocore/endpoint_provider.py	98.34% <98.34%> (ø)
awscli/botocore/compat.py	93.52% <100.00%> (+0.04%)	⬆️
awscli/botocore/exceptions.py	99.08% <100.00%> (+0.01%)	⬆️
awscli/botocore/utils.py	95.25% <100.00%> (+0.01%)	⬆️
awscli/s3transfer/tasks.py	91.66% <0.00%> (-3.34%)	⬇️

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[kyleknap]

Looks good 🚢