feat: persist the HTTP client inside of EasyPostClient

[Justintime50]

Description

We previously recreated the Guzzle Client on every request. This PR moves the creation of the HTTP client into the EasyPostClient object to ensure that back-to-back requests using the same EasyPostClient will reuse the same HTTP client to reduce memory consumption. There should be no user impact with this change.

Please note that although the HTTP client is now a publicly accessible property of the EasyPostClient, it is not intended to be replaced as no additional work has been made yet to allow this (see #318 for that initiative)

Closes #315

Testing

Re-recorded a cassette with two requests to ensure it works as expected

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)