[FEATURE] Add spec for data stream modify API and new parameter in restore API

[jainankitk]

Is your feature request related to a problem?

Two experimental additions in OpenSearch 3.8 need coverage in the API specification.

What solution would you like?

1. New endpoint:

• Path: /_data_stream/_modify, method POST.
• Query params: cluster_manager_timeout (time), timeout (time).
• Request body (required): object with an actions array. Each element is a
  single-key object keyed by the action type:
  • add_backing_index: { "data_stream": <string>, "index": <string> }
  • remove_backing_index: { "data_stream": <string>, "index": <string> }
• Response: acknowledged ({ "acknowledged": boolean }).
• Stability: experimental.

Reference (in-repo REST spec merged with the API PR): rest-api-spec/src/main/resources/rest-api-spec/api/indices.modify_data_stream.json

2. New param on restore snapshot body:

• Endpoint: POST /_snapshot/{repository}/{snapshot}/_restore.
• Add optional body field attach_to_data_stream (boolean, default false).
• When true, a restored .ds-<stream>-NNNNNN index is attached to a pre-existing data stream of the same name during the restore.

Do you have any additional context?

• Add API to modify a data stream's backing indices OpenSearch#22487
• Add option to attach restored backing indices to their data stream OpenSearch#22539