fix(type): add caster for SchemaType

[rap2hpoutre]

Summary

Adds caster in type definition.

According to this issue #10418, no concrete use case was found for adding caster in type definition files:

If there's a reason for them to be user facing I'm happy to add them to the TS bindings.

I actually found use case for plugins and introspection tools. I saw two examples in two different repositories (see examples below)

Examples

• https://github.com/ForestAdmin/forest-express-mongoose/blob/0bad7998b624d32a410464040001fa3f386ef49b/src/adapters/mongoose.js#L140-L143
• https://github.com/selego/mongoose-elastic/blob/dd318091230f1a10938347ebc5d0d8ea44e5bb6f/src/index.ts#L235-L240

It's my first contribution to mongoose, let me know if I can improve my PR.

[vkarpov15]

I think it would be better to put this property on Subdocument and DocumentArray schema types. Those are the only schema types on which this property is defined.

[vkarpov15]

One quick suggestion/discussion point: I don't think all schema types should have a caster property, only subdocs and document arrays. What do you think?

[rap2hpoutre]

@vkarpov15 Thank you for your answer! 🙏

I choose to put caster (optional) on SchemaType to be sure this kind of code will not trigger type error:

if (schema.paths[someRandomValue].caster.instance === "String") {}

I considered it would not be too noisy to add it on SchemaType. Still, maybe I'm wrong, if you feel like I should change my PR, I can change it 👍 (I'm not sure the code I submitted will be considered valid then).

[vkarpov15]

@rap2hpoutre I made an alternative fix in 7f49d40 - turns out caster isn't a SchemaType for DocumentArray, so I ended up having to add that property to both Array and DocumentArray