Add MySQL flexible Server cloud service

[ImMin5]

Description

We are planning to add MySQL flexible Server cloud service.
Because our MySQL Server cloud service only support Single server and Azure Database for MySQL single server is on the retirement path.

Cloud Service

• MySQL flexible Server

Release Version

• Undefined

[ImMin5]

@lhhyung

Hi, could you attach your PR or commit related to this issue?

[lhhyung]

• feat: Implement PostgreSQL Servers FlexibleServer type
• feat: Implement MySQL Servers FlexibleServer type