Introduce API instance pattern

[nkzou]

Refactors all api function calls to be part of an overall <name>API struct, which holds a configuration object. This reduces the amount of config passing needed.
Also introduces the with_http_info pattern so end users can choose to use a less verbose function call without details http response info.

usage example:

use datadog_api_client::datadog::configuration::Configuration;
use datadog_api_client::datadogV2::api::api_fastly_integration::{CreateFastlyAccountParams, FastlyIntegrationAPI};
use datadog_api_client::datadogV2::model::*;

#[tokio::main]
async fn main() {
    let configuration = Configuration::new();
    let fastly_api = FastlyIntegrationAPI::with_config(configuration);

    let params = CreateFastlyAccountParams {
        body: FastlyAccountCreateRequest {
            data: Box::new(FastlyAccountCreateRequestData {
                attributes: Box::new(FastlyAccountCreateRequestAttributes {
                    api_key: "fakesecretkey".to_string(),
                    name: "fastly_account_name".to_string(),
                    services: None,
                }),
                type_: FastlyAccountType::FASTLY_ACCOUNTS,
            }),
        },
    };
    let result = fastly_api.create_fastly_account(params).await;
    println!("{:#?}", result);
}