feat: support custom headers for sanctum client instance

[manchenkoff]

Is your PR related to a specific issue/feature? Please describe and mention issues.

Closes #95 by providing additional app.config.ts section to configure onRequest and onResponse interceptors for ofetch instance.

Additional context

Interceptor function receives an instance of NuxtApp, Logger, and FetchContext in order to have access to other plugins and configurations.

Checklist:

• [x] Code style and linters are passing
• [x] Backwards compatibility is maintained
• [x] Requires documentation to be updated