Rust SDK: Implement client hierarchy and sub-clients

[iamnamananand996]

Description

Replace the basic client struct with a complete hierarchy of clients and sub-clients.

Current Implementation

pub struct ApiClient {
    client: Client,
    base_url: String,
}

Target Implementation

pub struct ApiClient {
    config: ClientConfig,
    users: UsersClient,
    orders: OrdersClient,
    payments: PaymentsClient,
    notifications: NotificationsClient,
}

pub struct UsersClient {
    http_client: HttpClient,
}

pub struct HttpClient {
    client: reqwest::Client,
    config: ClientConfig,
}