Log diagnostic event if files not found in deployed payload

[brettsam]

This is another scenario that we should add to our "bad zipped directory" detection. A customer had deployed a zip that was somewhat correct, but we were unable to find any of the assemblies referenced in extensions.json due to a custom targets file that moved some files around during publishing. This was inadvertently copying these assemblies elsewhere and ended up with a structure like this:

|   extensions.json
|   functions.metadata
|   host.json
| ... assemblies
|   worker.config.json
|
+---.azurefunctions
|       function.deps.json
|       Microsoft.Azure.Functions.Worker.Extensions.pdb
|
+---bin
|   +---.azurefunctions
|   |       Microsoft.Azure.Functions.Worker.Extensions.dll
|   |       Microsoft.Azure.WebJobs.Extensions.FunctionMetadataLoader.dll
|   |       Microsoft.Azure.WebJobs.Host.Storage.dll
|   |       Microsoft.WindowsAzure.Storage.dll
|   |
|   ... some other folders/files

With those required folders being moved to /bin/.azurefunctions, we couldn't find them. The error is cryptic with:

Unable to load startup extension 'Startup' (Type: 'Microsoft.Azure.WebJobs.Extensions.FunctionMetadataLoader.Startup, Microsoft.Azure.WebJobs.Extensions.FunctionMetadataLoader, Version=1.0.0.0, Culture=neutral, PublicKeyToken=551316b6919f366c'). The type does not exist. Please validate the type and assembly names.

I propose that if we run into issues like this, we write a debug log with the customer's directory structure that allows us to quickly diagnose what they've done wrong. We should also emit a diagnostic event with a link to our docs (https://learn.microsoft.com/en-us/azure/azure-functions/dotnet-isolated-process-guide?tabs=hostbuilder%2Cwindows#deployment-payload)