Manually create Vulnerabilities via API

Notes to reviewers and maintainers

This MR is almost exactly the same as !61385 (merged) but here I added two things to address issues found in #10272 (comment 650202279):

1. ee/app/graphql/types/vulnerability_location/generic_type.rb which will return empty values for description field since we don't even expose that field for user input
2. ee/app/graphql/mutations/vulnerabilities/create.rb now wraps the errors in Array(), this is so that we don't end up calling to_a on a singular error message
3. Create a feature flag, rollout issue: #338694 (closed)
4. Remove requires_new: true from #transaction block due to #338730 (closed)

What does this MR do?

This MR adds an GraphQL query and underlying service object to create Vulnerability objects manually via API.

Related to #10272 (closed)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)