C# samples should be consistent in creating LuisApplication with either SubscriptionKey or AuthoringKey

[dmvtech]

Sample information

1. Sample type: any that use LUIS services
2. Sample language: C#
3. Sample name: below

Describe the bug

Some C# samples use SubscriptionKey and some use AuthoringKey from the bot file or config. This can cause issues when using same bot file across different samples (ex: core BasicBot versus webapi BasicBot). Mostly a problem when the different keys are associated with different regions. It should be consistent in implementation across samples.

The following samples use subscription key:
var app = new LuisApplication(luis.AppId, luis.SubscriptionKey, luis.GetEndpoint());

csharp_webapi\12.NLP-With-LUIS\App_Start\BotConfig.cs:90:
csharp_webapi\13.basic-bot\App_Start\BotServices.cs:54:
csharp_dotnetcore\21.luis-with-appinsights\Startup.cs:189:
csharp_dotnetcore\51.cafe-bot\BotServices.cs:40:

The following samples use authoring key:
var app = new LuisApplication(luis.AppId, luis.AuthoringKey, luis.GetEndpoint());

csharp_dotnetcore\12.nlp-with-luis\BotServices.cs:39:
csharp_dotnetcore\13.basic-bot\BotServices.cs:39:
csharp_dotnetcore\14.nlp-with-dispatch\Startup.cs:189:

Additional context

I didn't find any need to be using AuthoringKey over SubscriptionKey in the samples. But if there is, then obviously that would be a valid reason to keep it.

JavaScript flavors seem to go have an approach of going for SubscriptionKey alone or AuthoringKey if null:

endpointKey: luisConfig.subscriptionKey
OR
endpointKey: luisConfig.subscriptionKey || luisConfig.authoringKey,

[bug]

[dmvtech]

Mildly related: #1076