feat: add Surcharges (including category) to Rate model

[Justin Hammond (Justintime50)]

Description

Revives and rebases this PR onto current master and adds surcharge support on rates.

Changes in this PR:

• Adds Surcharges to the Rate model
• Adds Category to the Surcharge model (category JSON field)
• Moves the changelog note into the newest Next Release section at the top of the changelog
• Adds a unit test for surcharge category deserialization

Testing

• go test ./... -run TestRateUnmarshalSurchargeCategory -count=1 (compile/selection check)
• go test ./... -count=1 (fails in local environment due to missing test API fixtures/API key; unrelated to this change)

Pull Request Type

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc.)

[Justin Hammond (Justintime50)]

Closing this replacement PR to keep all discussion and review on #230 only.