[5.7] Add Builder::insertSub()

[miklcct]

This pull requests add the ability of inserting directly from a sub-query at SQL level, without loading data into memory.

Please refer to #2026 for its use.

[taylorotwell]

Is the syntax the same across all supported database drivers?

[miklcct]

They are the same in MySQL, SQL Server, PostgreSQL and SQLite.

Reference:
MySQL: https://dev.mysql.com/doc/refman/8.0/en/insert-select.html
SQL Server: https://docs.microsoft.com/en-us/sql/t-sql/statements/insert-transact-sql?view=sql-server-2017
PostgreSQL: https://www.postgresql.org/docs/9.1/sql-insert.html
SQLite: https://www.sqlite.org/lang_insert.html

[driesvints]

Use FQN here and proper spacing between parameters names

[taylorotwell]

Renamed to insertUsing.