[in_app_purchase] add Storefront.countryCode() and AppStore.sync()

[LouiseHsu]

Fixes flutter/flutter#159631 and fixes flutter/flutter#165659

Adds iap support for accessing store front country code and manual transaction syncing.

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or I have commented below to indicate which [version change exemption] this PR falls under[^1].
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style], or I have commented below to indicate which [CHANGELOG exemption] this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[hellohuanlin]

nit:

completion(.failure(error))
return

[LouiseHsu]

i changed it but whats the difference in practice?

[hellohuanlin]

Let's say we have a function that doesn't return:

func print(stuff: String) {
  ...
}

When we call it, we simply do:

if wannaPrint {
  print(stuff: "abc")
}

And if we want to early return, just do:

if wannaPrint {
  print(stuff: "abc")
  return
}

Now, what you were trying to do was:

if wannaPrint {
  return print(stuff: "abc")
}

For most languages, this code won't even compile. However, it compiles fine here in Swift, because Void is an actual type in Swift. So the above code is equivalent to:

if wannaPrint {
  let answer = print(stuff: "abc")
  return answer
}

Here the answer is of type Void, and of value Void(). While it's perfectly valid to write return Void(), I have never seen anyone writing it. People just do return for a function that returns void.

A fun fact, do you know thatVoid is a typealias of an empty tuple?

public typealias Void = ()

[LouiseHsu]

I see, that makes sense.

[hellohuanlin]

missing completion on success.

[hellohuanlin]

can it just be return _hostApi.countryCode()?

[hellohuanlin]

nit: Void() => ()