Optional middleware

[augustuswm]

• Updates all clients to 0.9.0
• Makes reqwest-middleware and related dependencies optional
• Introduces the middleware feature flag to control the inclusion of reqwest-middleware. This flag is enabled by default

Initial implementation for #93 . These changes only address the initial concern of forced dependency management by using these libraries. reqwest is still a required dependency, and additive feature request may still bring in the middleware feature.