feat: introduce DevelopersKnowledge

[bshaffer]

b/503382065

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• DevelopersKnowledge/samples/V1/DeveloperKnowledgeClient/batch_get_documents.php:25, tag developerknowledge_v1_generated_DeveloperKnowledge_BatchGetDocuments_sync
• DevelopersKnowledge/samples/V1/DeveloperKnowledgeClient/get_document.php:25, tag developerknowledge_v1_generated_DeveloperKnowledge_GetDocument_sync
• DevelopersKnowledge/samples/V1/DeveloperKnowledgeClient/search_document_chunks.php:25, tag developerknowledge_v1_generated_DeveloperKnowledge_SearchDocumentChunks_sync

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