[chore] Make base EasyPostService extendable outside module

[nwithan8]

Description

The EasyPostService base class used for all services (e.g. UserService) was marked internal, meaning it could not be constructed or extended outside of the EasyPost module. This PR instead sets it to protected, meaning it can still not be constructed directly, but can be inherited. This can be helpful for end-users trying to make extensions or custom implementations of the services, allowing them to use the base service rather than having to re-implement all client HTTP functionality themselves.

Testing

• All unit tests pass
• Change does not affect compilation, run-time or linting

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

[Justintime50]

Why not