Add new is_unknown flag in data model definition

[akugarg]

Add a new flag for licenses that are tagged as "unknown" i.e. not matched with a known named license.
This will help in segregating the special license keys with the known ones.

Short Description

Possible Labels

• new feature

Select Category

• Enhancement
• Add License/Copyright
• Scan Feature
• Packaging
• Documentation
• Expand Support
• Other

Describe the Update

How This Feature will help you/your organization

Possible Solution/Implementation Details

Example/Links if Any

Can you help with this Feature

[AyanSinhaMahapatra]

See #1675 for more details.