Use COPY in bulkCreate for Postgres

Issue Creation Checklist

• I understand that my issue will be automatically closed if I don't fill in the requested information
• I have read the contribution guidelines

Feature Description

Describe the feature you'd like to see implemented

I'm creating 700k+ records with bulkCreate() and running into the Postgres server being killed by the OOM killer on Linux.

Ideally large inserts would be done with COPY instead of INSERT.

Describe why you would like this feature to be added to Sequelize

It will improve the performance of bulkCreate()

Is this feature dialect-specific?

• No. This feature is relevant to Sequelize as a whole.
• Yes. This feature only applies to the following dialect(s): Postgres, I am not familiar with other dialects.

Would you be willing to resolve this issue by submitting a Pull Request?

• Yes, I have the time and I know how to start.
• Yes, I have the time but I will need guidance.
• No, I don't have the time, but my company or I are supporting Sequelize through donations on OpenCollective.
• No, I don't have the time, and I understand that I will need to wait until someone from the community or maintainers is interested in implementing my feature.

Indicate your interest in the addition of this feature by adding the 👍 reaction. Comments such as "+1" will be removed.

Start at the bulkCreate entry point and the PostgreSQL dialect handling, then determine how COPY could support large inserts. Clarify the behavior and compatibility requirements before implementation. Done means large PostgreSQL bulkCreate operations use COPY where appropriate and retain expected Sequelize behavior, with tests covering the new path.