Initial faiss interface support

[jmazanec15]

Description

This PR adds initial support through the knn_vector field type for faiss. This PR allows users to use HNSW method with flat encoding:

{
  "target-index" : {
    "mappings" : {
      "properties" : {
        "target-field" : {
          "type" : "knn_vector",
          "dimension" : 2,
          "method" : {
            "engine" : "faiss",
            "space_type" : "l2",
            "name" : "hnsw",
            "parameters" : {
              "m": 48,
              "encoder": {
                "name": "flat"
              }
            }
          }
        }
      }
    }
  }
}

In the future, we will add more methods.

The main changes include adding a MethodComponentContext parameter type, so that components such as encoders, may be configured through parameters, logic to build custom map of parameters including index description to be passed to faiss library, mapper interface changes, etc.

Additionally, added/refactored tests to better cover initial functionality.

Issues Resolved

#27

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed as per the DCO using --signoff

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[vamshin]

Minor comment. LGTM! Thanks

[VijayanB]

LGTM.