Add Elasticsearch KNN

[ceh]

Add an implementation for Elasticsearch KNN search. Fixes #298.

• Supports specifying index_options via arg groups, with one entry for Elasticsearch's default settings for M and EF.
• Supports specifying num_candidates via query args.

What do you think @erikbern, @alexklibisz?

[ceh]

Sample run for dataset fashion-mnist-784-euclidean on my local machine (M1 Pro):

Definitions file

float:
  euclidean:
    elasticsearch:
      docker-tag: ann-benchmarks-elasticsearch
      module: ann_benchmarks.algorithms.elasticsearch
      constructor: ElasticsearchKNN
      base-args: ["@metric", "@dimension"]
      run-groups:
        m-16-ef-100:
          arg-groups:
            - {"m": 16,  "ef_construction": 100} # index_options
          query-args: [[10, 20, 30, 40, 50]] # num_candidates
    opensearchknn:
      docker-tag: ann-benchmarks-opensearchknn
      module: ann_benchmarks.algorithms.opensearchknn
      constructor: OpenSearchKNN
      base-args: ["@metric", "@dimension"]
      run-groups:
        M-16:
          arg-groups:
            - {"M": 16,  "efConstruction": 512}
          query-args: [[10, 20, 30, 40, 50]]

[erikbern]

want me to merge this right now? i can re-run elasticsearch for the benchmarks (#399)

[alexklibisz]

LGTM, from a very quick skim

[ceh]

want me to merge this right now? i can re-run elasticsearch for the benchmarks (#399)

Nice! Fine to merge! Was thinking about adding a couple of run groups, but takes time to evaluate

[erikbern]

i'll probably re-run the benchmarks in the next few weeks anyway