Update JSON formatter to encode bytes as Base64 strings.

[aemous]

v1 port of #9424

Description of changes

• Modifies JSON formatter so that it serializes bytes into a UTF-8 Base64 string.
• Add a regression unit test to verify that binary data in the response gets base-64 encoded.

Description of tests

• Call a service operation using the CBOR protocol and receive a response containing a member with raw bytes. Prior to this change, error code 255 is observed and the full response is not printed. After the change, the output is fully printed, and it matches the output seen when using the JSON protocol on the same operation.
• Ran and passed all test suites and CI.

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.40%. Comparing base (5f07db1) to head (7d69ab8).
Report is 65 commits behind head on develop.

Files with missing lines	Patch %	Lines
awscli/utils.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #9438   +/-   ##
========================================
  Coverage    93.39%   93.40%           
========================================
  Files          210      210           
  Lines        16983    16986    +3     
========================================
+ Hits         15861    15865    +4     
+ Misses        1122     1121    -1     

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