Function build_key_package 

pub fn build_key_package(
    inbox_id: &str,
    credential: Credential,
    installation_keys: &XmtpInstallationCredential,
    provider: &impl OpenMlsProvider,
    options: KeyPackageOptions,
) -> Result<GeneratedKeyPackage, KeyPackageConstructionError>

Build a package with the supplied provider. Client bookkeeping is separate.