restore an ability to set z-threshold per node using network editor

[irenaby]

Pull Request Description:

z_threshold attribute has been previously removed from NodeActivationQuantizationConfig along with other attrs coming from QuantizationConfig. However we do want the ability to override it per node (currently via network editor). This PR adds z_threshold attribute back to NodeActivationQuantizationConfig with default value None. In activation qparams computation zthreshold is taken from QuantizationConfig unless the node config has a non-None value.

Checklist before requesting a review:

• I set the appropriate labels on the pull request.
• I have added/updated the release note draft (if necessary).
• I have updated the documentation to reflect my changes (if necessary).
• All function and files are well documented.
• All function and classes have type hints.
• There is a licenses in all file.
• The function and variable names are informative.
• I have checked for code duplications.
• I have added new unittest (if necessary).