Support for context params in service models

[jonemo]

Service model files (/botocore/data/*/*/service-2.json) will soon contain information about three types of "context params":

1. static context parameters, optional for each operation
2. (dynamic) context parameters, optional for each operation
3. client context parameters, optional for each service

This PR adds properties for accessing context parameters information in the ServiceModel class.

[codecov-commenter]

Codecov Report

Merging #2740 (43f620f) into develop (1504eb4) will decrease coverage by 0.01%.
The diff coverage is 95.83%.

@@             Coverage Diff             @@
##           develop    #2740      +/-   ##
===========================================
- Coverage    95.25%   95.24%   -0.02%     
===========================================
  Files           60       62       +2     
  Lines        12278    12869     +591     
===========================================
+ Hits         11696    12257     +561     
- Misses         582      612      +30     

Impacted Files	Coverage Δ
botocore/model.py	98.14% <95.83%> (-0.14%)	⬇️
botocore/signers.py	95.98% <0.00%> (-2.18%)	⬇️
botocore/configloader.py	97.91% <0.00%> (-2.09%)	⬇️
botocore/utils.py	93.59% <0.00%> (-0.02%)	⬇️
botocore/args.py	100.00% <0.00%> (ø)
botocore/tokens.py	86.41% <0.00%> (ø)
botocore/endpoint_provider.py	99.66% <0.00%> (ø)
botocore/credentials.py	98.65% <0.00%> (+<0.01%)	⬆️
botocore/exceptions.py	99.55% <0.00%> (+0.01%)	⬆️
botocore/client.py	98.74% <0.00%> (+0.01%)	⬆️
... and 6 more

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[nateprewitt]

Looks good, thanks!