Sort Sentry error list by frequency, last seen, and first seen

What does this MR do?

Overview

This MR adds sorting by frequency, last seen, and first seen to the backend api for the sentry issue index.

Technical Approach

The proposed approach is to request the issues from the Sentry api with a sort param.

Implementation Details

The sentry api can be queried using the following params

1. ?sort=freq which appears to search by frequency
2. ?sort=new which appears to search by firstSeen:
3. the default is lastSeen

Relevant controller action is here: render_index_json app/controllers/projects/error_tracking_controller.rb

Mapping of product requirements to Sentry api Fields

frequency => `count`
last_seen => `lastSeen`
first_seen => `firstSeen`

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

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 #34068 (closed)