fix(db): inherit generics in snapshotChanges, stateChanges, auditTrai…

[iNViTiON]

…l interface

valueChanges already support it (refer to #1214 ) but this one need change to @firebase
I submit separate PR to @firebase here firebase/firebase-js-sdk#590

Checklist

• Issue number for this PR: snapshotChanges does not inherit type of list/object #1523 (required)
• Docs included?: no
• Test units included?: no
• In a clean directory, yarn install, yarn test run successfully? (yes/no; required)

Description

link to issue #1214 ang PR firebase/firebase-js-sdk#590

Code sample

this.db.list<SampleModel>(`/users/${uid}`).snapshotChanges().subscription(a => {
const b = a[0].payload.val(); // b have SampleModel type
});

[jamesdaniels]

Needed more work, superseded by #1644 #1643

[iNViTiON]

@jamesdaniels look like @firebase willing to support generic type for DataSnapshot. Should we wait for their decision before extend it by ourselve?

[jamesdaniels]

@iNViTiON I'm fine implementing here as that pull request has been open for some time... paging @jshcrowthe