Fix the deserialization error of FieldEntry when the 'options' field …

[azerowall]

Currently, FieldEntry deserialization is done manually by using a custom deserializer. During the parsing process, if the "options" field appears before the "type" field, an error will occur.
Example of json on which the error occurs:

{
  "name": "title",
  "options": {
    "indexing": {
      "record": "position",
      "tokenizer": "default"
    },
    "stored": false
  },
  "type": "text"
}

This pull request changes the custom serializer to serde_derive macro, which fixes the described error.

[fulmicoton]

Thank you!