[V2] Enable support for loading files into nested parameters

[aemous]

Related Issues:

• Support fileb:// for nested objects #8013

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.

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[hssyoo]

I only had time to review the implementation for parsing the @= operator. I should be able to take a look at the implementation for quoted keys, tests, and the questions you had later this week.

Moving forward, we should try to cut separate PRs targeting a feature branch when implementing multiple features. IMO, support for shorthand paramfiles and quoted keys are related, but distinct features.

[aemous]

For reviewer, this change was made to avoid a circular import error.