Support function type as the default schema option

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Checklist

• Bugfix
• New feature
• Refactor

Related Issue

A function cannot be assigned to the default schema option, which makes it impossible to set dynamic values such as timestamps to a newly created document.

Solution

This PR provides the ability to assign a function to the default schema option. The function will be called and the returned value will be assigned to the document.

const userSchema = dango.schema({
  name: { type: 'string', required: true },
  createdAt: {
    type: 'date',
    default: () => Date.now(),
  },
});
const User = dango.model('user', userSchema);

const _id = await User.insertOne({ name: 'Jane' });
console.log(await User.findById(_id));

{
  name: "Jane",
  createdAt: 2022-08-22T10:52:30.699Z
}