feat(Decoder): Add config flag for merging into existing map values

[skaggmannen]

Description

This pull requests adds a config flag to prevent mapstructure.Decode() from clobbering existing values in nested maps in the destination. Instead, the incoming fields will be merged with existing fields in the destination.

Current behavior example

type Root struct {
	Nested Nested
}

type Nested struct {
	A int
	B int
}

src := A{ Nested: Nested{ A: 42 } }
dst := map[string]any{
	"Nested": map[string]any{
		"A": 123,
		"B": 456,
	},
}

if err := mapstructure.Decode(src, &dst); err != nil {
	panic(err)
}

// dst["Nested"]["A"] is now 42
// dst["Nested"]["B"] no longer exists

New behavior example

type Root struct {
	Nested Nested
}

type Nested struct {
	A int
	B int
}

src := A{ Nested: Nested{ A: 42 } }
dst := map[string]any{
	"Nested": map[string]any{
		"A": 123,
		"B": 456,
	},
}

dec, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
	MergeIntoExisting: true,
	Result: &dst,
})
if err != nil {
	panic(err)
}

if err := dec.Decode(src); err != nil {
	panic(err)
}

// dst["Nested"]["A"] is now 42
// dst["Nested"]["B"] is still 456