Add API call to convert channels attributes to nodes

[kwokcb]

Issue

It is possible to either miss or bypass the channels attribute on an input
If this occurs this can fail validation due to input / output type being found to be different types.

(Note: There also appears to be issues with how this attribute can sometimes not propagate properly
with interface inputs and outputs.)

Suggestion

Add a convenience utility to convert implicit channels attributes to explicit nodes (such as an extract,
swizzle etc)

[jstone-lucasfilm]

Thanks for this report, @kwokcb, and I'll just add a note that this will be addressed by the design changes in MaterialX 1.39, which includes the replacement of channels attributes with explicit nodes.