Helpshift SDK - UPM support

## Issue
Helpshift Unity SDK is missing unity's package manager support.

UPM standard requires things like:
- [Specific folder structure](https://docs.unity3d.com/Manual/cus-layout.html)
- Once UPM package is installed in the project its files are immutable by design. Meaning that SDK shouldn't ask users to edit its files.
- [Assembly definition files](https://docs.unity3d.com/6000.0/Documentation/Manual/assembly-definitions-creating.html)
- Package manifest (pacakge.json) in the root of the package folder

> [!NOTE]
> Having UPM support doesn't mean the SDK itself needs to be hosted in a npm registry. It can still be delivered in a .unitypackage format if that's still most convenient to you.

More info about it can be found here: [Unity Manual: Creating custom packages](https://docs.unity3d.com/Manual/CustomPackages.html)

### Benefits

This would be a great QoL improvement for developers. Managing project dependencies without it is a real pain.
UPM support exists in unity editor since 2017, and many SDK developers already adapted it. it's only natural to support it at this point.