Add option for using Client Credentials to download/seed a marketplace

[ThomPuiman]

Since the migration to the Sitecore Cloud Portal, the Portal credentials are no longer working for seeding or downloading a marketplace. I've added an option to create access tokens using API Clients that you can pass in.

The JSON file has the following structure:

[
    {
        "Name": "Sandbox",
        "OrderCloudBaseUrl": "https://sandbox.ordercloud.io",
        "ApiClientId": "00000000-0000-0000-0000-000000000000",
        "ApiClientSecret": "abcdefghijklmnopqrstuvwxyz"
    },
    {
        "Name": "Staging",
        "OrderCloudBaseUrl": "https://sandbox.ordercloud.io",
        "ApiClientId": "00000000-0000-0000-0000-000000000000",
        "ApiClientSecret": "abcdefghijklmnopqrstuvwxyz"
    }
]

It can have multiple environments/marketplaces in this file. Each operation can only target one environment.
The arguments added are:

• config - The path to the JSON file containing the configurations and API Client details
• target - The environment matching the Name property in the config file