Allow External Startup to update the config in all scenarios

Issue #6542 resolved a problem where if a functions app modified the configuration object (e.g. to retrieve configuration from KeyVault) compiling with Version 1.1.0 Microsoft.Azure.Functions.Extensions caused the error.

````
Microsoft.Azure.WebJobs.Script: The Functions scale controller may not scale the following functions correctly because some configuration values were modified in an external startup class.
````
Compiling with version 1.0.0 of Microsoft.Azure.Functions.Extensions works succesfullly.

The fix applied in #6542 only fixed this regression if the premium app service plan was being used AND runtime scale monitoring was enabled. It does not fix the problem:

- If some other plan is used (e.g. consumption)
- If runtime scale monitoring is not enabled

Thus myself and many other users who have commented on that closed issue are still unable to update to the latest version of the extensions assembly.

Start by reading issue #6542 and comparing its fix with the scenarios listed here: non-premium plans and apps without runtime scale monitoring. Verify the behavior with Microsoft.Azure.Functions.Extensions 1.0.0 versus 1.1.0. Done means external startup configuration updates work in both previously unsupported scenarios without producing the scale-controller error.

Written by the indexing model from the issue text.