[Backport 2.x] Added validations for indexPattern in autofollow API

[skumarp7]

Description

Backport https://github.com/opensearch-project/cross-cluster-replication/commit/6022720794ec37f7770d36d7a9909b61ecc056c6 from https://github.com/opensearch-project/cross-cluster-replication/pull/1373

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [ ] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license. For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[skumarp7]

Hi @ankitkala & @monusingh-1 ,

Please provide feedback :)