feat(database): add option to include key on valueChanges()

[sarunint]

Checklist

• Issue number for this PR: Retrieving list data with items id #1562 (required)
• Docs included?: not yet
• Test units included?: no? (original code has no test???)
• In a clean directory, yarn install, yarn test run successfully?: yes

Description

The RTDB version of #1976. Why should only Firestore have the fun?

Code sample

All of the following are valid.

this.afd.list('path').valueChanges();
this.afd.list('path').valueChanges(undefined, {idField: 'key'});

Caveats

Currently, if the list is an array of primitive types, it would not work. Please advise on how I should proceed.

[sarunint]

@jamesdaniels Whoa, I've already forgotten about this pull request 😂😂😂

[jamesdaniels]

Made it into 6.1.0-rc.1