Do not extend GuzzleHttp\Client

Problem/Motivation

PHPStan error we need to address before Guzzle v8. Currently this class is only "final" in its annotation, but will be truly final in v8.

Need to research the best way to design our clients around this restriction.

Ditto Regen-Digital/farm_loocc#15

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes