[feat] Add dedicated API Keys service

[nwithan8]

Description

• Add dedicated API Key-related service, available via the api_keys property of a client
  • NOTE: Please note the naming. The api_key property of a client is the currently-used API key string, while the api_keys property is the service for managing API keys.
• Migrate API key-related functions to API Keys service, deprecated in User service

Testing

• Migrate unit tests, re-record cassettes as needed

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)

Addressed