code / repository organization improvements : remove rt-xr-maf-plugin

[nlsdvl]

The unity player implementation currently comes as a set of 4 repos:

1. rt-xr-unity : reference player - unity project
2. rt-xr-gltfast : a fork of glTFast - unity plugin
3. rt-xr-maf-plugin : maf library & media pipelines - unity plugin.
4. rt-xr-maf-native : maf library & media pipelines - C++ libraries

This configuration makes the project more complex to maintain and reason about than it has to be.

Having fewer repos would make it easier:

• Most C# scripts in repo Hosting of the sample content #1 should be moved to be distributed through a unity package, not as project scripts.
• It seems the glTFast project has now become an official Unity package.
  All code related to parsing iso-23090-14 can be moved to the gltfast fork. But code related to a generic xr player implementation might not be relevant there.
• rt-xr-maf-plugin was initially intended to contain the maf implementation for unity. It currently only contains build artifacts and dependencies from rt-xr-maf-native. If media player code wrapping the MAF API for Unity is not maintained in this repo, it could be removed.

The project needs to be organized with the following in mind:

• the reference unity project should be buildable using Unity cloud build
• the code should be bundled as a unity package to be easily re-usable in custom unity projects

[nlsdvl]

rt-xr-maf-plugin is really just a build artifact of rt-xr-maf-native and there is no plan to add extra code or features there.
I propose to remove it. continuous delivery of rt-xr-maf-native and updated documentation should reduce the friction for new developpers.
cc @Etiennefaivredarcier @aaronmontilla

[Etiennefaivredarcier]

I agree on this, the number of repositories need to be reduced to invite new developers to test the tools

[aaronmontilla]

I completely agree with you, it was difficult for me to understand what each repository was used for. Reducing the number of repositories will make the first steps more intuitive for beginners.

[nlsdvl]

Progress on continous delivery for the media pipeline dependencies here: 5G-MAG/rt-xr-maf-native#6