Add endpoint to access resource iteration events

What does this MR do?

Adds an API endpoint to retrieve issue iteration events.

Database queries

/chatops run explain SELECT "resource_iteration_events".* FROM "resource_iteration_events" WHERE "resource_iteration_events"."issue_id" = 213 


Index Scan using index_resource_iteration_events_on_issue_id on resource_iteration_events  (cost=0.15..7.74 rows=5 width=50) (actual time=0.023..0.023 rows=0 loops=1)
  Index Cond: (issue_id = 213)
  Buffers: shared hit=4
Planning Time: 0.933 ms
Execution Time: 0.110 ms

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