[chore] Display object type, ID when converting object to string

[nwithan8]

Description

• Override ToString for EasyPost objects to display object type and optional ID

Testing

Pull Request Type

Please select the option(s) that are relevant to this PR.

• [ ] 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)
• [x] Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[jchen293]

Can we have a unit test to make sure the change is what we want?