severity and cvss metrics support

[chen-keinan]

This change is about supporting severity, CVSS v2, and v3 metrics (if exist)

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This PR (HEAD: 8503ad3) has been imported to Gerrit for code review.

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[gopherbot]

This PR (HEAD: 4d4b7f4) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/vulndb/+/330089 to see it.

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[gopherbot]

This PR (HEAD: a999515) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/vulndb/+/330089 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info