SPARKNLP-797: Introduce Protected Features

[DevinTDHa]

Description

This PR adds the functionality of Feature class parameters to be protected. This means, that the Feature can only be set once during the initialization process (for example loadSavedModel). Subsequent attempts to set it will result in an exception. The reason is, that some parameters of pretrained models should not be set after they have been loaded.

The following Feature parameters have been set to be protected:

• vocabulary
• merges
• signatures
• labels

Motivation and Context

Not letting the user set these parameters will result in less unintended behavior.

How Has This Been Tested?

Added new tests to verify the new behavior.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Code improvements with no or little impact
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING page.
• I have added tests to cover my changes.
• All new and existing tests passed.