Remove `FieldInfo.childFieldInfo`
Motivation:
We have introduced `TypeSignature` not to parse a struct type in place when creating `DocService` but put the struct in the list of `StructInfo` and link to it using `TypeSignature` in #422.
We could have the definition of a struct in only one place so we could dedup the information.
However, we have started to parse a bean(which is a struct) in place to represent a `@RequestObject` type in #1557.
To represent the type, `FieldInfo.childFieldInfo` was added and it turned out to be a bad decision.
Because after that, we have lots of duplicate information on structs. Also, the way that some of the structs use a link using `TypeSignature` whereas the rest of them don't, brought us the inconsistency of parsing and reorganizing the `DocService`.

Modifications:
- Remove `FieldInfo.childFieldInfos`.
  - `FieldInfo` now has only a `TypeSignature` and uses the `TypeSignature` to link to the corresponding struct.
  - The nested struct is not parsed in place.
- Add `REQUEST_OBJECT` `TypeSignature` for a `@RequestObject`.
- `FieldInfo`'s name for a `@RequestObject` is now the parameter name.
  - It was `class.getSimpleName` before which doesn't represent the field information properly.

Result:
- Unified way to create and organize `DocService`.