Support bulk insert in one query using newly added createAll() in connector

[samarpanB]

Suggestion

With the addition of efficient bulk insert in one DB query feature in loopback-connector after release of this PR, its important that we implement this method in postgres connector too as postgresql already supports insert into ... values ().. n dialect for multiple inserts in 1 single query.

Also relates to loopbackio/loopback-next#3357

Use Cases

Db query optimization. Faster insert. Less DB connection utilizations.

Examples

No changes needed in consuming apps if they are using existing createAll method of repository

Acceptance criteria

TBD - will be filled by the team.