Add IServiceProvider to dependency validator

#### What problem would the feature you're requesting solve? Please describe.
Customers are registering IServiceProvider, and this is leading to unexpected behavior and startup issues.

#### Describe the solution you'd like
Add validation logic such that customers running on:

- v3: receive a clear warning that this registration might cause startup issues.
- v4: receive a clear error stating that they cannot register IServiceProvider.

[Documentation](https://docs.microsoft.com/en-us/azure/azure-functions/functions-dotnet-dependency-injection#overriding-host-services) stating that functions doesn't support registration of host services already exists, and adding this layer of validation would ensure more clear errors indicating the problem.

#### Additional context
[DependencyValidator.cs](https://github.com/Azure/azure-functions-host/blob/dev/src/WebJobs.Script.WebHost/DependencyInjection/DependencyValidator/DependencyValidator.cs#L30)

Start with src/WebJobs.Script.WebHost/DependencyInjection/DependencyValidator/DependencyValidator.cs at the referenced validation logic, then review the linked dependency-injection documentation for the host-service restriction. Determine how registrations of IServiceProvider are detected for v3 and v4. Done means v3 produces a clear warning and v4 produces a clear error stating that IServiceProvider cannot be registered.

Written by the indexing model from the issue text.