Split Identifier into multiple structs

Follow-up of gitlab-org/security-products/analyzers/common!14 (diffs, comment 120272647)

CVEIdentifier could be a struct directly, instead of using Identifier everywhere. Identifier could also be an interface to simplify the code and ensure we can use all of the existing identifiers in functions.

Same goes go CWEIdentifier and others.

/cc @fcatteau @gonzoyumo