feat(entity): add mapOne

[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?

[ ] 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 #2538

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

If we agree on the API, I can update the docs.

[ngrxbot]

Preview docs changes for 2def007 at https://previews.ngrx.io/pr2628-2def0078/

[timdeschryver]

@brandonroberts do you got any ideas to make the build green? 😁

[brandonroberts]

😁 Add --skip-nx-cache to the build-affected command in the .circleci/config.yml file for now. That should cause a full rebuild of each package instead of using the cloud cache. Just temporary for now and I'll take a look for a better fix.

[brandonroberts]

Code LGTM. Need to add it to the docs page for the adapter

[timdeschryver]

@brandonroberts updated the docs, and I also exported EntityMapOne.