Flex Consumption plan support - Always Ready instances

**Is your feature request related to a problem? Please describe.**
Since upgrading to dotnet 8 isolated, my cold start has got substantially worse which is having a big impact on my customers. Ideally I'd like that fixing at source, but a new way of running Azure Functions has just been put into public preview which has always ready instances which would effectively fix the issue for me.

**Describe the solution you'd like**
On the standard license, be able to configure 1 or 2 always ready instances to stop the cold start issue affecting customers so much. I wouldn't expect it on the free one obviously. Even just 1 would be sufficient to be honest. I imagine this is possible currently by using the external functions features, but that comes with other config work - ideally I'd like to just be able to tick a box in the static web apps portal.

**Describe alternatives you've considered**
Fix the cold start issue on dotnet 8 isolated mode so that its back in a few seconds rather than 10+ seconds. Presume that's not within the domain of the static web apps team, so it feels like this solution would be more doable.