Don't embed embedded objects with valid JSON tags

[wolveix]

Fixes #975/#492.

If an embedded object has a json tag, it won't be embedded.

This example:

type Meta struct {
	CreatedAt time.Time `json:"created_at" db:"created_at"`
}

type GreetingOutput struct {
	Body        struct {
		Meta    `json:"meta"`
		Message string `json:"message" example:"Hello, world!" doc:"Greeting message"`
	}
}

Previously generated:

{
  "$schema": "https://example.com/schemas/GreetingOutputBody.json",
  "message": "Hello, world!",
  "created_at": "2019-08-24T14:15:22Z"
}

When it should be:

{
  "$schema": "https://example.com/schemas/GreetingOutputBody.json",
  "message": "Hello, world!",
  "meta": {
    "created_at": "2019-08-24T14:15:22Z"
  }
}

If no JSON tag is specified, it will still function as it did before (which is correct).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.12%. Comparing base (6d0295c) to head (1feda7d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #978   +/-   ##
=======================================
  Coverage   93.12%   93.12%           
=======================================
  Files          23       23           
  Lines        4768     4768           
=======================================
  Hits         4440     4440           
  Misses        269      269           
  Partials       59       59           

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[leonklingele]

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