feat: expose more information about the index

[heyjorgedev]

This PR adds new fields to the IndexInfo response.

"result": {
    "vectorCount": 7,
    "pendingVectorCount": 0,
    "indexSize": 43501,
    "dimension": 1024,
    "similarityFunction": "COSINE",
    "indexType": "HYBRID",
    "denseIndex": {
      "dimension": 1024,
      "similarityFunction": "COSINE",
      "embeddingModel": "BGE_M3"
    },
    "sparseIndex": {
      "embeddingModel": "BM25"
    },
    "namespaces": {
      "": {
        "vectorCount": 6,
        "pendingVectorCount": 0
      },
      "ns": {
        "vectorCount": 1,
        "pendingVectorCount": 0
      }
    }
  }

sparseIndex, denseIndex and indexType were added.