Add functionality to return sparse vectors in the FetchResponse

[honnuanand]

{
"vectors": {
"additionalProp": {
"id": "example-vector-1",
"values": [
0.1,
0.2,
0.3,
0.4,
0.5,
0.6,
0.7,
0.8
],
"sparseValues": {
"indices": [
1,
312,
822,
14,
980
],
"values": [
0.1,
0.2,
0.3,
0.4,
0.5
]
},
"metadata": {
"genre": "documentary",
"year": 2019
}
}
},
"namespace": "example-namespace"
}