Add support to shorthand syntax for loading parameters from files

[aemous]

Description of changes:

• Enable support for loading files into nested parameters via usage of file:// or fileb:// prefix.

Description of tests:

• Added unit test cases to ensure the parser works against the expected grammar updates.
• Added a test suite to verify that the parser correctly loads contents from local files as expected.
• Ran and passed all existing and new tests.
• Manually tested the feature via self-crafted input.

Usage Example: For example, suppose there is a file named tagval.txt located at path/to/tagval.txt on the host machine with contents TagValue.

aws codecommit tag-resource --resource-arn <RESOURCE_ARN> --tags TagKey@=file://path/to/tagval.txt

The above command will parse the tags parameter as "tags": {"TagKey": "TagValue"}}. Notice the need for the assignment operator @=. This operator is required to prevent breaking existing customers that may be using a nested parameter value that happens to start with file:// already.

Relevant Links:

• v2 PR

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 28 lines in your changes missing coverage. Please review.

Project coverage is 0.08%. Comparing base (cdea504) to head (f21aa57). Report is 16 commits behind head on develop.

Files with missing lines	Patch %	Lines
awscli/shorthand.py	0.00%	26 Missing :warning:
awscli/paramfile.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #9063 +/- ## ========================================== - Coverage 0.08% 0.08% -0.01% ========================================== Files 210 210 Lines 16955 16977 +22 ========================================== Hits 14 14 - Misses 16941 16963 +22 ```

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