feat(data-classes): decode base64 encoded body

[michaelbrewer]

Issue #, if available:

Description of changes:

Code example:

def lambda_handler(event: dict, context):
    event = APIGatewayProxyEvent(event)
    # Dynamically base64 decode body as a str
    body: str = event.decoded_body
    print(body)

Where the event is:

{
  "body": "W3siaWQiOjEsIm5hbWUiOiJDb21wYW55IE9uZSJ9LHsiaWQiOjIsIm5hbWUiOiJDb21wYW55IFR3byJ9XQo=",
   "isBase64Encoded": true
}

Print statement:

[{"id":1,"name":"Company One"},{"id":2,"name":"Company Two"}]

• Based on a tweet: https://twitter.com/donkersgood/status/1390292767662825473

Checklist

• Meet tenets criteria
• Update tests
• Update docs
• PR title follows conventional commit semantics

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #425 (7183448) into develop (22754d3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #425   +/-   ##
========================================
  Coverage    99.90%   99.90%           
========================================
  Files          102      102           
  Lines         4117     4124    +7     
  Branches       204      205    +1     
========================================
+ Hits          4113     4120    +7     
  Misses           1        1           
  Partials         3        3           

Impacted Files	Coverage Δ
...lambda_powertools/utilities/data_classes/common.py	100.00% <100.00%> (ø)

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[heitorlessa]

Minor change and it's good to merge and release 🎉