RUMM-671 DatadogObjC target is fixed for SPM

[buranmert]

What and why?

DatadogObjC wasn't compiling with @objc attribute used without importing module 'Foundation' errors
This only happens in Swift Package Manager; other dependency managers work fine
For more info: #218

How?

I added import Foundation lines to source files.
I also made dep-manager-test/spm import DatadogObjC instead of Datadog, this should test both targets at once.
Note: I'm still investigating how come Xcode manages to build the project without those imports 🤔

Release info

This PR is branched off from 1.3.0 tag and to be merged into master
We will ship this change as a hotfix once it is merged

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

[ncreated]

Have you tried adding

linkerSettings: [
   .linkedFramework("Foundation")
]

to the DatadogObjc target in Package.swift? If it solves the problem, this should be IMO preferred solution. Otherwise, we could easily miss import Foundation in any new file added to the DatadogObjc and the issue will be back there.

[ncreated]

Release info

This PR is branched off from 1.3.0 tag and to be merged into master
We will ship this change as a hotfix once it is merged

If we plan to include two patches in a hotfix, please create the hotfix/1.3.1 branch and point both PRs to it. This way we'll have two merge commits, keeping the git history right. Otherwise, both patches will be included in a single, RUMM-669 merge commit, which is not correct.

[buranmert]

@ncreated .linkedFramework("Foundation") didn't work and produced the same errors.
also it's mostly for C++ based libraries and non-modular libraries according to docs and only very few projects use it
we cannot easily miss that because otherwise dependency-manager-tests-spm will fail ❌

it's weird that Xcode can compile the project, when i create new project it fails when import Foundation is absent 🤔

UPDATE: hotfix/1.3.1 is here and fixes have it as their target branch ✅

[ncreated]

it's weird that Xcode can compile the project, when i create new project it fails when import Foundation is absent 🤔

I guess it may depend on the Xcode version used to create the project.