Add filtering to VulnerabilitiesFinder

What does this MR do?

1. This MR adds the following filters to VulnerabilitiesFinder and Vulnerability:

• by project
• by report type
• by severity
• by state

2. It adds Group#vulnerabilities, which returns vulnerabilities for all projects in a group and its subgroups

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This is preliminary work for replacing vulnerability findings with first class vulnerabilities on the security dashboards. That work can be followed at https://gitlab.com/groups/gitlab-org/-/boards/1229233?scope=all&utf8=%E2%9C%93&state=opened&label_name[]=first%20class%20vulnerabilities.

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