Add created_at and updated_at filters to Epics API

What does this MR do?

Accessing recently created and recently modified data is an important access pattern, but is not easy using the Epics API. Although is is possible to manually halt one sufficiently new data is found, pagination headers give no useful feedback about the amount of data a client needs to consume.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/11242

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