feat(transaction): add transaction support

[shubhamp-sf]

Description

• Provides beginTransaction method to SequelizeCrudRepository with somewhat similar usage as loopback transactions.
• It can have different isolation levels similar to loopback.
• Timeout parameter support is not there in this PR.

Fixes #21

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Tested with postgresql and sqlite3 database.

• Tested if retrieves model instance once transaction is committed.
• Tested if it can rollback transaction.
• Ensured that created transactions are isolated based on the isolation level passed.
• Ensured local transactions. (i.e. It should not use transaction with another repository of different datasource).
• Tested in a freshly created lb4 app https://github.com/shubhamp-sf/loopback4-sequelize-transaction-example

Checklist:

• Performed a self-review of my own code
• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• Add usage details in README.md.

[sfdevops]

🎉 This PR is included in version 2.2.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀