Added public function client.newSchema to create a new XtkSchema object from an XML document or element…

[mkiki]

Description

Working with workflow temp schemas in XML form can be tedious and this PR is about being able to create a XtkSchema object from any XML representation of a schema, including workflow temporary schemas. The newSchema free function exists but is private. It is now publicly exposed as client.newSchema

How Has This Been Tested?

New unit tests have been written.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.