Refactor TokenPlugin to use createClientPlugin

[anwarpro]

I refactored TokenPlugin to use the createClientPlugin factory function from Ktor. This change addresses a runtime IR linking error encountered on iOS.

The TokenPlugin's functionality remains the same:

• It adds a token to request headers.
• Configuration includes setting a token header name and a token provider.

I also added unit tests for TokenPlugin to verify:

• Correct token insertion into headers.
• Handling of null tokens from the provider.
• Validation of plugin configuration for null tokenHeaderName or tokenProvider.

(AI Generated)