[EPIC] Composite primary/foreign keys

Nobody has claimed this yet.

_See https://github.com/strongloop/loopback/issues/2080 for the original discussion in LB 2.x/3.x._

LoopBack should support models using composite keys, where a primary key or a foreign key is composed from multiple properties.

Let's consider the following models as an example:

```ts
// A (GitHub) repository is identified by the pair [user, name]
// and has many issues
@model
class Repository {
@property({id: true})
user: string;

@property({id: true}
name: string;

@hasMany(() => Issue)
issues: Issue[];
}

// An issue belongs to a repository
@model
class Issue {
// how to specify @belongsTo relation?
}
```

Functional areas we need to cover:

- How to define a model with a composite primary key (e.g. Repository above)?
- Connectors must understand composite ids and convert them to a concept the target database understands. SQL is easy because it supports composite primary keys OOTB, NoSQL like MongoDB and Cloudant can be tricky.
- create a new model
- find a model by id
- update (patch/replace) a model by id
- delete a model by id
- (etc.)
- Autoupdate/automigrate need to support composite primary keys too
- How to define composite foreign key in a model belonging to another model (e.g. Issue above)
- Relation repositories must work with composite keys
- How to expose a model with a composite primary key via REST API?
- How to expose a relation using a composite foreign key via REST API?
- Controller templates need to produce code that works with composite primary/foreign keys.

Begin with the linked LB 2.x/3.x discussion and the Repository/Issue model example. Scope the work across model definitions, connectors, automigrate/autoupdate, relation repositories, REST exposure, and controller templates; done requires agreed support for each listed operation and a testable implementation plan.

Written by the indexing model from the issue text.