Extract formatter for container scanning report vulnerabilities

What does this MR do?

This MR extracts Gitlab::Ci::Parsers::Security::Formatters::ContainerScanning from the container scanning parser in order to:

1. Encapsulate logic specific to the formatting of container scanning vulnerabilities, as has already been done for DAST vulnerabilities
2. Allow the reuse of this functionality in when updating project fingerprints to our standardized verson https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14848

This MR paves the way for the work needed to complete https://gitlab.com/gitlab-org/gitlab-ee/issues/10561

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team