feat: add support for multiple database connections

[davelsan]

Issue

This PR addresses graphql-server-model-codegen issue#143.

Description

This issue synchronizes graphql-server with code generator changes in branch issue#143-multiple-databases. The goal of this PR is to add support for arbitrary local database connections defined in the data models layer.

See Work Notes for detailed information.

Blocks

• None. But it needs to be merged along with graphql-server-model-codegen#146.

Notes

BREAKING CHANGES

• This PR invalidates the sequelize configuration in config.json. A new data_models_storage_config.json config file has been added. The new config file is used to perform migrations and seeding to the appropriate database.

Semantic Versioning

• This is an API change. This package might have to be bumped to the next minor version.

[davelsan]

Fixed pending issues and bumped to version 0.3.0.

Should be ready to merge along with graphql-server-model-codegen#146.