Support default values for custom types by performing the default scan before decoding

[markbradley27]

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update
• Go Version Update
• Dependency Update

Description

Rather than setting defaults on the output of the decode, this sets defaults on the input. This means that all of the fancy logic in decode that handles custom converters and TextUnmarshallers can apply to default values as well.

DRAFT: Still needs some error handling updates, encode support, and general polish, but sending this out as a draft to demonstrate that the approach works. You can see that the unit test still passes for all of the previously supported default types, as well as a new custom type. I'll try to get around to polishing this up soon.

Related Tickets & Documents

• Closes [FEATURE] default values for custom types #225

Added/updated tests?

• Yes
• No, and this is why: please replace this line with details on why tests
  have not been included
• I need help with writing tests

Run verifications and test

• make verify is passing
• make test is passing