Add support for Flex Consumption plans

[jamesharling]

Is your feature request related to a problem? Please describe.
Currently this deployment only uses the regular consumption plan for its App Service Plan - this older plan does not support any advanced networking integrations. By moving to, or optionally supporting, Flex Consumption plans it would be possible for all of the resources to require private endpoint access - ideal for something as sensitive as certificate management.

Describe the solution you'd like
The deployment template migrates to using flex consumption plans, or has an option to opt in to them (as they are preview only at the moment). Additional configuration should be exposed to create private endpoints and/or network injection.

Describe alternatives you've considered
None that apply.

Additional context
n/a

[shibayan]

Duplicate #624