Public GlobalTransform API

## What problem does this solve or what need does it fill?

Bevy docs and the GlobalTransform API gives off the impression that GlobalTransform should exclusively be managed by Transform. But this philosophy is inconsistent and not great for some use case.

* The transform pipeline is opaque and difficult to work with for alternative coordinate systems.
* GlobalTransform can only be constructed by a few `From` methods.
* There is a warning for having a parent without `GlobalTransform`, but that is in fact fine if managed by an alternative API.

## What solution would you like?

Add function constructors to GlobalTransform.

Explicitly support third party crates editing and managing GlobalTransform. Remove or change the warning.

## What alternative(s) have you considered?

Add marker traits to opt out of the transform pipeline.
Add an alternative GlobalTransform component for third party crates.