[Relation] Supports HasOne relation

[dhmlau]

Description / Steps to reproduce / Feature proposal

Add support for HasOne relation between two models.

Acceptance Criteria

• Update factory method to handle this relation
• create interface for CRUD methods available for the relation. See this as an example.
  • create default class which implements the interface
• Implement the hasOne decorator
  • model decorator: infers the metadata
• create test cases
• update docs
• create a follow up issue to demonstrate using multiple relations in a new/existing example

[dhmlau]

Cross posting in #1449 (comment) from @elv1s:

We have a requirement for hasOne in addition to hasMany and belongsTo

Currently hasOne is not part of our Q1 roadmap. So I have 2 questions:

1. @elv1s, what is the timeline you're thinking in terms of having hasOne support?
2. @strongloop/sq-lb-apex @raymondfeng @bajtos, after we have the 2 model relations, do you think it's a relatively small effort for us to get hasOne in?
   EDIT: I realized that we've estimated this task already. Given what we know now (esp when implementing BelongsTo, are we still good with this estimate?

[elv1s]

We are developing a POC with LB3 right now. If we move to LB4 we would need hasOne relationships early 2019.

[b-admike]

Closing as done.