feat(types): support casting JSON form of buffers

[Fonger]

Summary
resolve #6863

Support the following use case with Buffer
Now Buffer works with string, array, Binary(from bson) and object (see the example)

const Model = mongoose.model('Foo', new Schema({ buf: Buffer }));
// in document
let doc = await Model.create({
  buf: {
    type: 'Buffer',
    data: [ 229, 143, 176, 231, 129, 163 ]
  }
});
assert.equal(doc.buf.toString('utf8'), '台灣');
// in query
doc = await Model.findOne({
  buf: {
    type: 'Buffer',
    data: [ 229, 143, 176, 231, 129, 163 ]
  }
})
assert.equal(doc.buf.toString('utf8'), '台灣');

Test plan

• cast on querying
• cast on creating model (also add cast from binary, string, array)

[vkarpov15]

Great, thanks!