Replace Newtonsoft.Json with System.Text.Json

[anthonie-kramer]

IN PROGRESS NOTES:

• STJ does not like abstract classes... We need more Converters to handle deserialization
• STJ does not like methods in Constructors (neither does Newtonsoft, so I'm not sure how we get away with it now)

BACKGROUND:

• Trying to replace Newtonsoft.Json with System.Text.Json for performance improvements

DESCRIPTION:

• Merges Ian's previous solution (branch stj) with the most recent master branch
• Additional Converters are required due to deeper abstractions that were added (like BoundedCurve)

TESTING:

• Ensure that the tests in the project are passing
• Publish a function that uses the updated Elements to ensure it works with Explore

FUTURE WORK:

• We will also need to update CodeGen stuff
• Update other projects that use Newtonsoft.Json

REQUIRED:

• All changes are up to date in CHANGELOG.md.
• This is compatible with the CLI
• This is compatible with Explore

COMMENTS:

• Any other notes.

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