feat(issueVariant): Allow for issue variant creation with just severity rating

[BlakePatterson]

Task Description
Currently the severity input type for creating issue variants only accepts a cvss vector. Add a rating field alongside vector and update the resolver methods to allow for creating a new issue variant where vector is not provided.

Acceptance Criteria:

• Add a "rating" field of type SeverityValues to SeverityInput
• Add a new "NewSeverityFromRating" function that creates a severity with only the rating field and not the vector
• Add logic to handle between when a vector is provided and when only a rating is provided
• Be able to create a new IssueVariant with only a rating

[lolaapenna]

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