appservice: Implementation for flex deployment using new api

[nturinski]

Flex deployment requirements changed quite a bit. I'll try to break it down as succulently as possible.

• Remote build should be defaulted. The only runtime where it should be true is python, I believe. I am getting confirmation from the team.
• Because we shouldn't prompt for remote build anymore (we used to ask the user if they wanted it or not), I removed the customDeployment message logic and reverted it back to how it used to be.
• Flex deployment API endpoint changed. It required the headers for it to accept the request now as well.
• Similar to my Functions PR, the payload for the old API version isn't compatible with flex deployment (I need a property that only exists on the new payload). That's why I'm doing the get request
• For flex deployment, we need to create a storage blob container. The endpoint takes care of it from there. We're able to get connection strings and etc. from the functionAppConfig

[MicroFish91]

LGTM!

I'll try to break it down as succulently as possible

Can confirm, PR was extra succulent 👍

[MicroFish91]

Nit: extra new line