refactoring: Standardize format of search params in engine configs

[tellet-q]

1. Address: Standardize format of search params in engine configs #119

Make search_params consistent across all the engines. The new format is:
"search_params": [ { "parallel": 1, "config": { ... } }, ... ],

2. Fix milvus benchmark execution by upgrading milvus image version to milvusdb/milvus:v2.3.1

[KShivendu]

Great start.

I realised that [{search_params: { parallel: 100, config: {...} } }] would be better than [{search_params: { parallel: 100, params: {...} } }]

Wdyt? If you agree, please change accordingly.

[KShivendu]

LGTM. Please confirm that you've tested all the engines once and then we can merge this.

[tellet-q]

@KShivendu I have tested all the engines once with laion-small-clip dataset.

[KShivendu]

Great. I've tested the engines from my end too 🚀

[joein]

this pr is breaking backwards compatibility with the previous versions, isn't it?

[KShivendu]

We discussed and agreed that it's okay.