User defined alert fingerprinting

What does this MR do?

This uses the fingerprint param to set the fingerprint in the AlertManagement::Alert.

If we encounter an alert with the same fingerprint as another of the Project's alert, we increment the events count instead of creating a new alert object. This part will close #217050 (closed).

Currently I'm always hashing the given fingerprint to SHA1, as the sha_attribute concern was designed to work with a SHA1 hash (according to the comments in the file).

Screenshots

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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Part of #214556 (closed)