Core: add variant builder implementation

[aihuaxu]

This PR adds the implementation to build a variant for primitive, object and array.

The usage:

• Build from Json
  VariantBuilder.parseJson(json_string);

• build a primitive

VariantBuilder builder = new VariantBuilder();
VariantPrimitiveBuilder primBuilder = builder.createPrimitive();
Variant variant = primBuilder.writeNumeric(10).build();

• build an object

 VariantBuilder builder = new VariantBuilder();
 VariantObjectBuilder objBuilder = builder.startObject();
 objBuilder.writeString("key1", "value1").writeNumeric("key2", 3).endObject();
 Variant variant = builder.build();

• build an array

 VariantBuilder builder = new VariantBuilder();
 VariantArrayBuilder arrBuilder = builder.createArray();
 arrBuilder.writerNumeric(10).writerNumeric(20).endArray();
 Variant variant = builder.build();

Nested arrays and objects can be supported as well.

Part of: #10392