GH-3540: Allow user-provided embeddings in VectorStore

[aniketg-21]

🚀 Motivation

Current VectorStore implementations (e.g., ChromaVectorStore) automatically generate embeddings from Document.content using the configured EmbeddingModel. However, more advanced use cases require greater flexibility:

1. Precomputed embeddings: Embeddings may be generated externally, such as via batch pipelines or fine-tuned models.
2. Embedding semantic intent: In some cases, embeddings should reflect a prompt, summary, or intent—not the raw content.
3. Structured data: When storing structured formats (e.g., JSON), embedding the full raw structure often reduces semantic quality.

🔧 Enhancement

This PR introduces support for user-supplied embeddings, allowing applications to decouple document storage from semantic representation. This separation enables more accurate and intentional embedding strategies for search and retrieval.

• If no embedding is provided, the default behavior—auto-generating embeddings from content—is preserved.
• This change improves semantic search relevance without compromising the integrity or structure of stored documents.

✅ Backward Compatibility

Fully maintained. The default pipeline continues to auto-generate embeddings when none are explicitly provided.

🧪 Testing & Limitations

This feature has been tested with SimpleVectorStore. Other implementations (e.g., Pinecone, Milvus) were not tested due to environment constraints and may require further validation or adapter adjustments.

✅ Issue Closed

Closes #3540