Not declaring generated ProtoGrpcType inheritance

### Problem description
When using proto-loader-gen-types for ts-types generating, the root `ProtoGrpcType` type is not declared to `extends grpc.GrpcObject`.
Because of that, when I `grpc.loadPackageDefinition(...)`, I cannot cast it directly to `as MyProtoGrpcType` and I have to cast it to `as unknown` first.
Though root `ProtoGrpcType` is in fact the extension of `grpc.GrpcObject` -- the only problem is the absence of declaration.

### Reproduction steps
```typescript
import * as grpc from '@grpc/grpc-js';
import * as protoLoader from '@grpc/proto-loader';
import type { ProtoGrpcType as MyProtoGrpcType } from 'proto/my';

const myPackageDefinition = protoLoader.loadSync('my.proto', { includeDirs: [ 'proto' ] });
const myProtoDescriptor = grpc.loadPackageDefinition(myPackageDefinition) as unknown as MyGrpcType;
```

### Environment
- Any OS name, version and architecture
- Any Node version
- Any Node installation method (nvm or others)
- @grpc/proto-loader:0.7.13

### Additional context
@murgatroid99 , I strongly appreciate your creature: the proto-loader-gen-types!
It is a very helpful thing! And I even can go on with `as unknown` cast.
But this duct-tape of `as unknown` can be removed -- and please let's do it.
I created a PR addressing this problem, it's very simple. Please take a look.