KV - Add option to add properties to targetField instead of completely override it

[deepforest]

Given a 'kv' processor, It's kind of essential to be able to add matching keys to the targetField, if exists, instead of completely override it. For example:

{
    "kv": {
      "config": {
        "field": "kv",
        "fieldSplit": "\\|#",
        "includeKeys": [
          "key1",
          "key2",
          "key3"
        ],
        "targetField": "@params",
        "targetFieldMergeStrategy": "Replace" | "Concat" <-- suggestion
      }
    }
}

In the example above, given a log such as:

{
  ...
  "message": "My log message|#key1=value1|#key2=value2|#key3=value3",
  "@params":
    "my": "param"
}

the final result shoud be:

{
  ...
  "message": "My log message|#key1=value1|#key2=value2|#key3=value3",
  "@params":
    "my": "param",
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
}

currently we are getting:

{
  ...
  "message": "My log message|#key1=value1|#key2=value2|#key3=value3",
  "@params":
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
}