feat(Schematics): add ng add for @ngrx/entity

[timdeschryver]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

Adds the ng add schematic to @ngrx/entity.

It will only install the entity package and add it to the package.json.

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #920

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

With this, we've covered all of the modules to have a ng add command.