Support multi_match function in SQL/PPL query engine

[chloe-zh]

Related design is presented in issue #182

Todo list

• Support the multi_match functionality by pushing it down to the search engine
• Enable multi_match function in SQL and PPL syntax and parser, including all the available parameters
• Make sure the function in new engine is compatible with the old engine support (including match_match and multimatch functions)
• Add unit tests. (Unit tests are mandatory for all code changes.)
• Add integration test cases for multi_match
• Update user manual

Function details

The multi_match function maps the multi match query used in the search engine. This query builds on the match query to allow multi-field queries.

Syntax:
multi_match([field_list], query_expression[, option=<option_value>]*)

Available options:

• analyzer
• auto_generate_synonyms_phrase
• boost
• operator
• minimum_should_match
• lenient
• zero_terms_query
• type_breaker
• cutoff_frequency
• type (best_fields, most_fields, cross_fields, phrase, phrase_prefix, bool_prefix)

Sample queries:

# Search query 1
GET my_index/_search
{
  "query": {
    "multi_match" : {
      "query":    "this is a test", 
      "fields": [ "subject", "message" ] 
    }
  }
}

# SQL
SELECT * FROM my_index WHERE multi_match([subject, message], "this is a test")

# PPL
source=my_index | where multi_match([subject, message], "this is a test")


# Search query 2
GET my_index/_search
{
  "query": {
    "multi_match" : {
      "query":      "brown fox",
      "type":       "best_fields",
      "fields":     [ "subject", "message" ],
      "tie_breaker": 0.3
    }
  }
}

# SQL
SELECT * FROM my_index WHERE multi_match([subject, message], "brown fox", type="best_fields", tie_breaker=0.3)

# PPL
source=my_index | where multi_match([subject, message], "this is a test", type="best_fields", tie_breaker=0.3)

[acarbonetto]

fixed by Bit-Quill#71 and #649