support json tags in embedded structs

[jgillich]

Describe the PR
This change makes it so json tags on embedded structs are honored.

One example of this:

type RevValueBase struct {
	Status bool `json:"Status"`

	Err int32 `json:"Err,omitempty"`
}
type RevValue struct {
	RevValueBase `json:"rev_value_base"`
}

Previously, RevValueBase was inlined into RevValue. With this change, it matches the behavior of the core json package and the embedded struct becomes a field.

"rev_value_base": {
    "$ref": "#/definitions/web.RevValueBase"
}

It should be noted that this changes the schema generation of embedded structs. However, since it was impossible to unmarshal embedded structs previously, I don't believe this is could cause breakage unless people implemented custom de-serialization.

Relation issue
Fixes #886

[sdghchj]

LGTM， But I'd like to follow @ubogdan 's opinion for this feature.

[codecov]

Codecov Report

Base: 95.10% // Head: 95.03% // Decreases project coverage by -0.06% ⚠️

Coverage data is based on head (11a00d5) compared to base (29d3d30).
Patch coverage: 85.71% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1396      +/-   ##
==========================================
- Coverage   95.10%   95.03%   -0.07%     
==========================================
  Files          15       15              
  Lines        3062     3064       +2     
==========================================
  Hits         2912     2912              
- Misses         86       87       +1     
- Partials       64       65       +1     

Impacted Files	Coverage Δ
parser.go	94.05% <81.81%> (ø)
field_parser.go	97.44% <100.00%> (-0.56%)	⬇️

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[ubogdan]

LGTM

[ubogdan]

LGTM， But I'd like to follow @ubogdan 's opinion for this feature.

As soon as it doesn't break older contract I'm fine.

[ubogdan]

@jgillich Thanks for your contribution.