fix(afs): optional get() options, get() for docs, and zone work

[jamesdaniels]

Checklist

• Issue number for this PR: #nnn (required)
• Docs included?: (yes/no; required for all API/functional changes)
• Test units included?: (yes/no; required)
• In a clean directory, yarn install, yarn test run successfully? (yes/no; required)

Description

• get()'s options should be optional
• Add get() to docs
• get() should return to the Angular zone
• Since it's Promise-based and doesn't have side-effects, create and use a new simplified runInZone operator

Code sample

[davideast]

LGTM