Added Autogenerated File Header and Renamed Client

[saimedhi]

Description

• Added a header to indicate that this file is autogenerated in generation templates.
• Updated the client name in all files from "Elasticsearch PHP Client" to "OpenSearch PHP Client".

Issues Resolved

Related to https://github.com/opensearch-project/opensearch-php/pull/194#discussion_r1618729256, #195

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[saimedhi]

@dblock,

Could you please review and merge this if it looks good?

Thank you!