Better type inference for egg-mongoose

Hi!, We're trying to use egg.js in production with mongoose, but we notice a problem about the typing for Mongoose with `egg-ts-mongoose` plugin.

Mongoose itself has the typing solution(separate interface declaration or use `ts-mongoose`), and the `egg-ts-helper` also generated a correct type file of model files(interface `IModel`), Awesome!
But when using the model in service by `this.ctx.model`, the model type is referred to the `egg.Context.model` in `egg-ts-mongoose/index.d.ts`(https://github.com/eggjs/egg-mongoose/blob/master/index.d.ts#L28), which type is `[key: string]: mongoose.Model`.

So in the service, the model type is `Model`, it's very stupid! and the solution also very easy, just override the type of `context.model` to `IModel`, by edit `typing/index.d.ts` as below:
```typescript
import 'egg';

declare module 'egg' {
// add below lines
interface Context {
model: IModel
}
}
```
(Edit the `typing/model/index.d.ts` should has same result)

I think this job can not be done by `egg-mongoose`, because the interface `IModel` is a generated file.

I'm using WebStorm 2020.2, but I think the VSCode should has the same problem, because the `context.model` type refer to `egg-mongoose` seems a correct behavior or maybe I wrong :D

Thanks!

Start with egg-ts-mongoose/index.d.ts and the generated typing/index.d.ts or typing/model/index.d.ts described in the issue. Trace how egg-ts-helper generates IModel, then verify the service this.ctx.model type; done means generated declarations expose the model-specific type instead of Model.

Written by the indexing model from the issue text.