Add support for tenant-specific endpoints

[JadeDickinson]

What

• Adds support for tenant-specific endpoints by adding the class MarketingCloudSDK::ExactTargetEndpoints
• This reads in tenant-specific values provided in the application using this gem.
• Since these vary by organisation these can no longer be hardcoded in this gem.
• Therefore this feature requires applications using this gem to add a file to their root directory, ./config/endpoints.yml, with the same keys as config/endpoints.yml from this gem.
• Developers must then replace the placeholder values given with their tenant-specific endpoints for (SOAP)[https://help.salesforce.com/s/articleView?language=en_US&type=1&id=000356497] and (REST)[https://help.salesforce.com/s/articleView?id=000356498&type=1] endpoints.

Why

Addresses https://github.com/salesforce-marketingcloud/FuelSDK-Ruby/issues/123 and makes this gem usable from the end of September 2022.