@loopback/cli: Controller rest generator with findOne method

@mrnetforge is already working on this.

Since May 24, 2020.

## Suggestion

Currently, cli generator controller doesn't provide findOne method inside file. Method findOne exists in repository that is associated with controller, so i think i should be generated. For now i must manually add this method to all my controllers

```ts
@get('/config/findOne', {
responses: {
'200': {
description: 'Config model instance',
content: {
'application/json': {
schema: getModelSchemaRef(Config, { includeRelations: true }),
},
},
},
},
})
async findOne(
@param.filter(Config) filter?: Filter,
): Promise {
return this.configRepository.findOne(filter);
}
```

## Use Cases

`lb4 controller` with REST option should create findOne inside controller (example above)

## Examples

In my scenario i have model that i need to find not by id but based on custom filter. It would be much easier when this method can be created out of the box.

## Acceptance criteria

TBD - will be filled by the team.