Create VulnerabilitiesSummaryType for GraphQL

What does this MR do?

• Adds Vulnerability.counted_by_severity
• Adds VulnerabilitiesSummaryType present vulnerability counts in GraphQL
• Adds vulnerabilities_summary field to ProjectType
• Adds VulnerabilitiesSummary pseudo-model
• Adds VulnerabilitiesSummaryPolicy

This MR will provide the data for #207442 (closed)

TODO

• Add first_class_vulnerabilities feature flag to vulnerabilities_summary
• Update GraphQL docs and schema
• Add specs for VulnerabilitiesSummary
• Add specs for VulnerabilitiesSummaryPolicy

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.