Declare metadatas in Sdr registry

[sebastienblor]

Changes proposed in this pull request
This PR declares some arnold metadatas as Sdr info in the registry.
The list of supported metadatas are the following one
USD builtin metadatas :

• page : gives the UI group to put this attribute under. It's based on the arnold metadata "ui.groups"
• label : UI label of the parameter. Based on arnold attr metadata "label"
• help : description of the parameter. Based on existing "desc" arnold metadata
• connectable : whether this parameter is connectable or not. Based on existing "linkable" metadata. Note that in Arnold it defaults to true, so it won't be set for a lot of parameters that are actually connectable. To be verified if this is similar in USD or if we should always declare it
• role (returns the shader classification/category)

Custom metadatas (declared as "hints")

• uimin : based on arnold "min" metadata
• uimax : based on arnold "max" metadata
• uisoftmin : based on arnold "softmin" metadata
• uisoftmax : based on arnold "softmax" metadata

Enums:
enum attributes now declare all the allowed enum values in the "options" field

Issues fixed in this pull request
Fixes #55

[cpichard]

typo attributres -> attributes