feat: Set default values for all addressable structs

[gitsang]

Set default values for all addressable structs when struct fields has default tag.

type OtherStruct struct {
  Name string `default:"hello"`
}

type Sample struct {
  Maps map[string]OtherStruct
}

func main() {
	obj := &Sample{
		Maps: map[string]OtherStruct{
			"hello": {},
		},
	}
	if err := defaults.Set(obj); err != nil {
		panic(err)
	}
}

Previously, in this situation, it would output {"maps": {"hello": {}}} and the default value would not be set to OtherStruct.

After modification, it will output {"maps": {"hello": {"name": "hello"}}}

I don't know if this is the expected behavior, but I think the default value should take effect anywhere, which is more intuitive.

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.05%. Comparing base (abebf4b) to head (b5a101c).

Files with missing lines	Patch %	Lines
defaults.go	78.57%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   95.97%   96.05%   +0.07%     
==========================================
  Files           2        2              
  Lines         149      152       +3     
==========================================
+ Hits          143      146       +3     
  Misses          3        3              
  Partials        3        3              

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