Extracted from #9763

Let the Podfile be managed by the flutter tool at build time rather than as an artifact of flutter create. This lets us version Podfiles created by old invocations of flutter create as well as handle upgrades of hybrid Podfiles that users have altered to include their own pod dependencies. Probably by tagging the file with markers such as

User stuff
User stuff
####################
# Auto-generated v0.2
# Do not edit within the marked lines. They may be replaced or removed automatically
our own generated pod script
# You can edit below this marked line
####################

This way, users also can't run pod install in the iOS folder before using plugins or editing his own Podfile and get a strange error.

With this, we can also pod deintegrate the project if no plugins are left and there are no entries outside our marked generated lines.