refactor: use shared function to initialize models

[lamkeewei]

Problem

When adding/removing a model, developer need to remember to update both test and runtime loaders.

Solution

Improvements:

• Introduce shared initializeModels function to be used in both test and runtime sequelize loaders.

Bug fixes

• Load Agency model for tests

[miazima]

Is this something we could extract out too to prevent inconsistencies?

postmangovsg/backend/src/core/loaders/sequelize.loader.ts

Lines 60 to 68 in f4c90e8

	// Create the default credential names in the credentials table
	// Each name should be accompanied by an entry in Secrets Manager
	await Promise.all(
	[
	DefaultCredentialName.Email,
	formatDefaultCredentialName(DefaultCredentialName.SMS),
	formatDefaultCredentialName(DefaultCredentialName.Telegram),
	].map((name) => Credential.upsert({ name }))
	)

[lamkeewei]

Is this something we could extract out too to prevent inconsistencies?

postmangovsg/backend/src/core/loaders/sequelize.loader.ts

Lines 60 to 68 in f4c90e8

	// Create the default credential names in the credentials table
	// Each name should be accompanied by an entry in Secrets Manager
	await Promise.all(
	[
	DefaultCredentialName.Email,
	formatDefaultCredentialName(DefaultCredentialName.SMS),
	formatDefaultCredentialName(DefaultCredentialName.Telegram),
	].map((name) => Credential.upsert({ name }))
	)

Yep definitely. But I'm keeping this here first because I think using the seeders from #1168 will be more appropriate for this purpose.