[chore, feat] Send FedEx SmartPost registration to correct endpoint, prep carrier account parameters

[nwithan8]

Description

• Fix FedEx SmartPost not going to the /register endpoint when creating a carrier account.
• Add custom parameter set for FedEx SmartPost (alongside FedEx and UPS)
• Add enums for common carrier account types under CarrierAccountType
  • Replaces FedExAccount and UpsAccount constants that were introduced only in the previous release (no concern for breaking change issue)
• Adds constructors for carrier account creation parameter sets that take in carrier account type as an argument
  • Old way (no argument) labeled as deprecated, as we want to move users towards defining the type during parameter construction
  • This will also come in handy if/when we introduce parameter sets for all carrier accounts
• Adjust the relationship between base classes for carrier account creation parameters to remove unnecessary parameters (e.g. CreateFedEx no longer has a useless Credentials property, Create no longer has a useless RegistrationData property)

Testing

• Update unit test to account for changes to constants, re-record cassettes as needed
  • Minimal-to-no changes to unit tests suggest this will not be a breaking change for end-users (this alters unadvertised code that was only introduced a week ago anyway)

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)