[Feature] Support PostgreSQL database type

[kiddkevin00]

The development for this feature includes the following tasks:

• Update Connection Pool class and support a default connection defined in "package.json" file.
• Create a new Store class.
• Add test cases for all the introducing functionalities.

Acceptance criteria:

• Able to connect to local PostgreSQL database.
• Able to perform basic operations, including CRUD and configuring index (following the implementation of Mongo Store class).
• The test coverage should not drop because of the introducing functionalities.