Query nested relations recursively

Nobody has claimed this yet.

## Suggestion

Original from [comment](https://github.com/strongloop/loopback-next/issues/3454#issuecomment-609847868).

LoopBack 4 provides the custom scope to achieve inclusion, which was implemented in #3454. But it only goes down to the certain levels of relations that are described in the filter, for example, three levels:
```
const filter = {
include: [
{
relation: 'nested 1',
scope: {
include: [{relation: 'nested 2'}],
},
},
],
};
```
It'd be good to have a better way to query nested relations to max depth/ N degrees, something like `{relation: "nested", depth: 10}`.

## Examples

Model `Comment` has many `replies: Comment[]`, and comments in the `replies` has many `replies`, etc. Returns when there is no more replies or the max depth is reached.

## Acceptance criteria

- [ ] Provide a way to query nested relations to max depth/ N degrees, or at least to have a helper function for writing the filter.
- [ ] Should document how to configure the max depth of such traverses.

Start by reading the custom-scope inclusion work in issue #3454 and the nested-relation filter examples in this issue. Define how a depth-limited traversal should behave for recursive Comment replies, then document the max-depth configuration and verify that traversal stops at either the limit or the end of the relation.

Written by the indexing model from the issue text.