feat: when enum not found, default to unspecified error code on serialization

[jpill]

This change introduces a custom converter that populates a default unspecified error code if the json value cannot be found in the enumeration on serialization.

This will allow additions to be made to the error code enumeration in the ShipEngine code without immediately causing serialization exceptions here, which will help prevent unnecessary disruptions.

[coveralls]

Pull Request Test Coverage Report for Build 8818963923

Details

• 13 of 13 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 85.33%

---

Totals
Change from base Build 8787017493:	0.1%
Covered Lines:	661
Relevant Lines:	754

---

💛 - Coveralls

[brock-bouchard]

Is there a similar change we need to make in the other SDKs?

[jpill]

Is there a similar change we need to make in the other SDKs?

At the very least I'll go through and update with the current error codes we have in each, and where I can determine similar serialization may cause errors will look to account for those as well.