Swift Package Manager support

[onevcat]

This pull request contains support for the newly added Swift Package Manager (SPM) support of Xcode 11.

From Xcode 11, it supports to build a library against UIKit with SPM. That means we can use it to provide an easier way to integrate our SDK into user's project. I modified some of the project files/settings to let this project compatible with SPM. There is also a new sample app target LineSDKSample(SPM) to verify the behavior when building with SPM.

Some concerns and things not contained yet:

1. Since the SPM now does not support building resource yet, I removed all resource related things out. For example, the LoginButton and all localization files, and the new sharing UI planned to be added in next version. It is proposed in this file, maybe we can see it next year.
2. Only the LineSDK target now supports SPM. Our ObjC wrapper LineSDKObjC is not yet. I wonder whether it worths or be possible to do so. Since the ObjC wrapper is also a Swift project and it should be just a subset of using Swift features, it is possible to also provide SPM support for it. However, users of this wrapper is using our SDK through Objective-C, I don't know whether users can get any benefit from SPM when using Objective-C as their language.
3. The SPM support from Xcode 11 would add some new sections to the project file, especially to the LineSDKSample project (due to a new target using SPM is added). It is now not a back compatible change, so it breaks the project structure in Xcode 10. Users cannot check and run the sample project if using Xcode 10 after this gets merged. So I believe at least we should wait for Xcode 11 officially release before we can merge it.
4. We should also add swiftLanguageVersions and testTarget in the Package.swift to make it better.

@rmundo @eJamesLin Can you check and try it in your environments to verify the correctness of these implementation? Thanks!

[eJamesLin]

👍
Successfully verified integration using both SPM and CocoaPods.

Kind of pity that user could not open the sample project by Xcode10 once merged.

[onevcat]

I guess we will not merge it until Xcode 11 officially released. But fortunately it just affect the sample project. Carthage users can still build the LineSDK.framework target without problem even using Xcode 10.

[onevcat]

@eJamesLin I removed the sample app using SPM, so it now can be opened without problem in Xcode 10 too. I guess it would be a good time to merge it since it is a better starting point for the binary framework feature.