[FEATURE] Api-based signing via payment service - Android

[lealobanov]

Implement Android sign method for API-based signing, accepting transaction and bytes parameters.

See iOS native method:
https://github.com/onflow/FRW-iOS/blob/5bc4094f1898646e184d3f2026c3c5e5162f8d13/FRW/Services/Manager/Config/RemoteConfigManager.swift#L214-L223

[lealobanov]

Data models for FCLVoucher and SignPayerRequest/SignPayerResponse are not yet implemented on Android, may be a good idea to add these to flow-kmm and use them universally throughout the project.