[ML] Adding placeholder functionality for custom model request logic

[jonathan-buttner]

This PR adds logic to serialize java objects into JSON values and implement the placeholder replacement logic. The Custom service was leveraging gson to handle the serialization into json. This implementation uses our internal XContent provider instead.

This is needed for the Custom service functionality in the inference API. The Custom service will support PUT requests with placeholders (${some_key}). The ValidatingSubstitutor handles replacing the placeholders with the values passed in via a map.

Example PUT Request

PUT _inference/sparse_embedding/custom_sparse_embedding
{
  "service":"custom-model",
  "service_settings":{
    "secret_parameters":{
      "api_key":<<your api_key>>
    },
    "url":"http://url",
    "headers":{
      "Authorization": "Bearer ${api_key}",
      "Content-Type": "application/json;charset=utf-8"
    },
    "request":{
      "content":"""
        {
          "input": ${input},
          "input_type": "${input_type}",
          "return_token": ${return_token}
        }
        """
    },
    "response":{
      "json_parser":{
        "token_path":"$.result[*].embeddings[*].token",
         "weight_path":"$.result[*].embeddings[*].weight"
      }
    }
  },
  "task_settings":{
    "parameters":{
      "input_type":"query",
      "return_token":true
    }
  }
}

In this example ${input_type} would be replaced with query and ${return_token} would be replaced with true.

[jonathan-buttner]

Both of these are needed for the Apache commons StringSubstitutor being used for replacing the placeholder.

[jonathan-buttner]

This handles the quoting for us, for example a string will be surrounded by quotes.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[prwhelan]

Wouldn't this just be substitutedString.indexOf('$') != -1? Or do we allow instances of $ (like at the end of a string)?

[jonathan-buttner]

Yeah we'd have to look for ${ I suppose, but yeah I think the idea would be to let a string have any number of $ throughout it or even ${ (as long as it didn't have the closing }).

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x