RUMM-655 Add lyft/Kronos dependency for NTP time sync

[ncreated]

What and why?

📦 We want to use NTP to compute the real time of writing events (logs, spans, rum). In this PR I'm adding the lyft/Kronos dependency which does the NTP sync. It is not yet used in SDK code, as I wanted to separate project configuration from the actual work in SDK. This PR is not targeting the master branch yet.

How?

Cocoapods :

# Datadog.podspec + Datadog.podspec.src
s.dependency 'Kronos', '~> 4.1'

SPM:

// Package.swift
.package(url: "https://github.com/lyft/Kronos.git", .upToNextMinor(from: "4.1.0"))

Carthage required few changes to the project (and the update in dependency-manager-tests/carthage/CTProject). As it was quite strenuous, I'm leaving the note on what was changed (it's also written in commit messages):

Things change fast in Carthage (their guides are outdated), but this is how it worked in Xcode 12.0 (12A7209), target by target:

• Datadog:
  • Add Kronos.framework to Datadog > Build Phases > Link Binary With Libraries.
  • Add Datadog > Build Settings > Frameworks Search Path: $(SRCROOT)/../Carthage/Build/iOS
• Example:
  • Add Kronos.framework to Example > Frameworks, Libraries and Embedded Content.
  • Add Example > Build Settings > Frameworks Search Path: $(SRCROOT)/../Carthage/Build/iOS
• DatadogIntegrationTests:
  • Add 'Copy Files' build phase, coping Kronos.framework into Frameworks folder
• DatadogTests, DatadogBenchmarkTests:
  • no changes

Also, I had to exclude the arm64 architecture for iOS Simulator builds (done through .xcconfig) due to getting:

error: module 'Kronos' was created for incompatible target arm64-apple-ios8.0

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