RDART-964: Add PrivacyInfo.xcprivacy for iOS and macOS

[nielsenko]

Adds privacy manifests to our iOS and macOS plugins.

While we only need to update the plugins listed here for now (ie RealmSwift), the wording of the page:

The following are commonly used SDKs in apps on the App Store

suggests that the list of things for which this is required is just an arbitrary cutoff rather than a conceptual distinction, so it seems safest to just assume the list will grow over time and do this for the realm flutter package as well. To be honest I don't trust that Apple got the list right.

Note that we are following the standard Flutter / Cocoapods implementation which has the following caveat:

The large caveat is that we do not currently know if this actually works. This is the method of inclusion that seems to be the consensus among people using Cocoapods, as bundling it directly as a resource causes problems for clients who do not use use_frameworks. (In theory it seems like a manifest would not actually be required in that case since there is no framework, but it has the potential to actually stomp top-level resources.) Hopefully the automated analysis that Apple will eventually roll out will tolerate the file being bundled in a resource bundle in the framework rather than a top-level manifest file. If not, however, it's not clear how Cocoapods can be supported, so we can adopt this common approach for now under the assumption that eventually tooling will adapt to the reality of the ecosystem, and revisit the exact bundling later if necessary.

Source: flutter/packages@c5349bc

The flutter repo is tracking the approach here.

As to why the we (and the flutter team) add _privacy to the resource bundle id see
flutter/packages#5846 (comment)

The actual privacy manifest files are lifted from the realm-swift repo.

I'll quote the commit message, as to. why it looks as it does:

Since we don't include the analytics code in builds for the app store, the only
thing we need to declare is that we call fstat() on files in the app container,
which core uses internally to check the size of Realm files. This is assuming
that if the app logs into Atlas that's something that the app itself has to
declare rather than us, as we don't automatically talk to the server on our own
behalf.

The CocoaPods bundling might not actually work. This is the approach that most
of the SDKs on the mandatory privacy manifest list are taking, but since Apple
isn't actually checking the manifests yet no one knows if they'll accept this
fairly weird way of bundling the xcprivacy file. The less weird way breaks
building pods as non-framework static libraries.

Fixes: #1551
Fixes: #1629
Fixes: #1630

[nirinchev]

Looks good, should we symlink those manifests or duplicating them is fine?

[nielsenko]

Looks good, should we symlink those manifests or duplicating them is fine?

I would prefer not, for now.