appservice: Update steps and validation to support new domain name label scopes

[MicroFish91]

Here are the updated steps and validation for supporting the new domain label scopes, see here for more context.

New validation required for site names:

In general, here are the checks/validation that should be done depending on the customer’s selection:
 
* If global hostname selected (old format of “*.azurewebsites.net”), use these three (3) checks/validations prior to allowing customer to create site:
  1.      Global CheckNameAvailability (CNA) API (the one without providing region)
  2.      Regional CheckNameAvailability (CNA) API (the one where you need to provide region)
  3.      ARM ID Check / Check if a resource already exists with the same ARM ID
    a.      A “create” on a pre-existing resource with same ARM ID would result to an “update” of the site resource
    b.      We do this check by doing a GET on the site and if it results in:
        i.      Status 200 – this means that there is already a pre-existing site with the exact ARM ID, so you would need block user from creating the resource
        ii.      Status 404 – this means that no resource exists with the exact ARM ID, so you can allow user to create the resource
    
  Special case: If site name input (user input) is >=47 chars, ignore result of regional CNA because it inherently has a shorter character limit than Global CNA

* If regional hostname selected (new format of “*.<RegionName>.azurewebsites.net”), use these two (2) checks/validations prior to allowing customer to create site:
    1. Regional CNA
    2. ARM ID Check / Check if a resource already exists with the same ARM ID
 
"Global CNA" is the global (no region/location in the parameter) CheckNameAvailability API
"Regional CNA" is the regional (with region/location in the parameter) CheckNameAvailability API

Corresponding vscode-azureappservice PR: microsoft/vscode-azureappservice#2703

Edit: After some experimentation, I've found that selecting for the Tenant domain label scope actually fails if over 43 chars long, even though the validation says it's okay. This is the error:

Setting the limit to 43 chars instead seems to completely fix the issue. Verified portal has a similar limit.