Add support for functions max scale out limit

[MartinWa]

The changes in this PR are as follows:

• Adds max_scale_out_limit to set a maximum number of workers that a site can scale out to in consumption and Elastic Premium Plans

I have read the contributing guidelines and have completed the following:

• [x] Tested my code end-to-end against a live Azure subscription.
• [x] Updated the documentation in the docs folder for the affected changes.
• [x] Written unit tests against the modified code that I have made.
• [x] Updated the release notes with a new entry for this PR.
• [x] Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here: I do not currently have access to Azure to test this but I have done so in the past.

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

let myFunction = functions {
    name "myWebApp"
    sku WebApp.Sku.Y
    max_scale_out_limit 5
}

[Larocceau]

I've done a successful end-to-end test on this.