Call to validate_query in custom metrics form is not retried

What does this MR do?

Custom metrics form does not retry after validate query request hits a 202. The expectation is to retry because the backend queues requests and caches results that are only available for subsequent calls (more info here #13135 (comment 231228162)).

Additionally, a loading state is introduced because the backOff calls to validate queries take longer time.

Screenshots

Before	After

Screenshot with Validating query... text

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

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

Closes #13135 (closed)