feat: add change_message attribute to field content

[njausteve]

Title

Upgrade typedstruct dependency and add JSON encoding for FieldContent struct

Type of Change

• New feature
• Refactoring

Description

This pull request includes the following updates:

• Migrated from the unmaintained typed_struct to the maintained typedstruct package to ensure future support and compatibility.
• Added JSON encoding functionality to the FieldContent struct in the ExPass.Structs.FieldContent module. The new encoder converts FieldContent struct data into camelCase keys and filters out nil values. This is critical for ensuring consistent interaction with external systems via APIs.
• Introduced a new attribute change_message in the FieldContent struct, which allows descriptive messages when field values change.

Testing

• Updated test cases for FieldContent to include validation of the change_message field.
• Added tests for the new JSON encoding functionality, including key formatting (camelCase) and exclusion of nil values.

Impact

• Dependencies: Migrating to typedstruct may introduce minor changes in struct behavior, but no breaking changes are expected.
• External APIs that consume FieldContent JSON data should now receive properly formatted camelCase keys, ensuring compatibility with standardized systems.
• Developers must now ensure that change_message follows the format requirements (%@ placeholder).

Additional Information

N/A

[njausteve]

closed: #146