[BUG] Pull-based ingestion fails with remote cluster state due to missing XContent serialization

[SwethaGuptha]

Describe the bug

Starting with OpenSearch 2.17, remote cluster state publication is supported. The experimental pull-based ingestion feature(#17442) introduced an ingestionStatus field in IndexMetadata(#17631). However, this field is only serializable via byte streams and is missing from IndexMetadata.toXContent() https://github.com/opensearch-project/OpenSearch/blob/main/server/src/main/java/org/opensearch/cluster/metadata/IndexMetadata.java#L2163-L2269. The XContent object is used in:

1. /_cluster/state rest API - to fetch the cluster state
2. Remote cluster state is serialized and de-serialized as XContent.

Due to the miss, nodes in clusters using remote store for cluster state publication cannot receive ingestionStatus updates, breaking pull-based ingestion functionality.

Related component

Indexing

To Reproduce

A 3.1 cluster.

Expected behavior

Add XContent Serialisation

Additional Details

No response

[SwethaGuptha]

@varunbharadwaj @yupeng9 Please take a look at this.

[varunbharadwaj]

Thanks for catching @SwethaGuptha. Will take a look.