Added SPM support (#156)

[cabmunich1985]

Description

As Flutter is not supporting cocoapods any longer on the long run it is required to make this framework SPM compatible. Therefore this change allows anyone to use aepsdk_flutter via SPM. In the same moment Cocoapods support is dropped.

Related Issue

Issue #156

Motivation and Context

We currently have a lot of red information on ending support of cocoapods within our Flutter builds. aepsdk_flutter is the last framework missing, to fully migrate to SPM and drop all Cocoapods related pieces.

How Has This Been Tested?

With the fork I tested against our own infrastructure, replacing the old used frameworks with the fork to ensure that the integration works as expected.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

As change was breaking I made a major bump to the versions to 6.x.x.

Checklist:

• I have signed the Adobe Open Source CLA. (Not yet, need to clarify)
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

I think adding tests is not useful for this kind of change (there were no tests for Cocoapods either)