RUMM-1744 Copy Kronos code to SDK

[ncreated]

What and why?

📦 This PR copies MobileNativeFoundation/Kronos@5259250 code directly to our SDK and removes the link to Kronos in all dependency managers.

There are few motivations to use our own version of Kronos:

• We want to collect diagnostic and telemetry data from Kronos code (in our dogfood projects) to analyse and act on Kronos dependency triggers a Local Network Permission alert on iOS >= 14 #647 issue.
• Own version gives us more control over this code - this especially important if we want to support platforms x dependency managers which are not supported in upstream Kronos.
• Embedded code means less dependencies to our SDK users - faster installation (e.g. carthage bootstrap) and simpler linking for their app runtimes.

On the other hand, we shouldn't lose much by dropping external dependency on Kronos. It is well sustained project, solving only one problem (NTP sync) and not requiring significant updates nor bugfixes (ref.: release history). We will track all updates to the original repo with applying critical patches. Also, if we manage to find & fix issues in original implementation, we should contribute to the upstream project.

How?

• Recent Kronos code and tests (MobileNativeFoundation/Kronos@5259250) were copied to our repository - everything was put into separate folder.
• I removed Kronos dependency from Datadog.podspec, Cartfile and Package.swift.
• For clarity, I prefixed all imported types and files with Kronos*.
• Changed public access control to internal for respective types.
• Adjusted code style to pass our linter.
• Added licensing header in each file, referencing original authors.

Next PR(s):

I found two problems, that will be addressed in separate PR(s), to keep this one small and clear:

• Some of imported tests are very flaky as they do real UDP. I want to look closer into this and see what we can do.
• I will update our distribution tool to assert Kronos.xcframework presence only in SDK versions below 1.8.x.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference

[maxep]

All good 👍

[buranmert]

this is actually a breaking change for our carthage users.
ideally this would require a major version update.
we should at least communicate this in release notes.

[ncreated]

Indeed 👍, Carthage users will have to remove Kronos.xcframework from their list of linked libraries, otherwise it will fail on first clean build. Although, I don't think it's a breaking change requiring major update (well, it's Carthage limitation), but it's definitely something we need to mention under "Migration Steps (Carthage)" in Release Notes for 1.9.0 👌.

[ncreated]

Added needs-migration-steps-explained label.