add quilt3.get_boto3_session() function

[sir-sigurd]

Description

This provides public API for creation of boto3 session with stack credentials/region.
This also should reduce number of S3 requests (1 per non us-east-1 bucket (assuming buckets are in the stack region)) 🚀.

TODO

• Unit tests
• Documentation
  • Python: Run build.py for new docstrings
• Changelog entry (skip if change is not significant to end users, e.g. docs only)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.80%. Comparing base (229e313) to head (c5c6932).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4169      +/-   ##
==========================================
+ Coverage   37.73%   37.80%   +0.07%     
==========================================
  Files         766      766              
  Lines       35152    35195      +43     
  Branches     5190     5190              
==========================================
+ Hits        13264    13307      +43     
  Misses      20656    20656              
  Partials     1232     1232              

Flag	Coverage Δ
api-python	91.09% <100.00%> (+0.05%)	⬆️
catalog	12.12% <ø> (ø)
lambda	87.95% <ø> (ø)

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[nl0]

lgtm