Support Azure Database for MySQL

[elemount]

Related issue: #806

Description

This PR will support gh-ost run on Azure Database for MySQL. Like GCP or Aliyun RDS, Azure Database also need to do some specific things with code modification. This PR including 3 things

1. Skip port validation for Azure Database for MySQL. It acts as GCP and Aliyun RDS.
2. Add document for how to use it on Azure Database for MySQL.

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[elemount]

@shlomi-noach , would you mind to review this PR?

Thanks,
elemount

[timvaillancourt]

@elemount these changes look good but I don't have cycles to test on Azure

Can you confirm this works in tests on a real Azure database?

[ArtemTrofimushkin]

Hello, @elemount, @timvaillancourt
Will be this RP merged to main branch?
This fork allows us to apply schema changes on all ours Azure Managed MySQL Database instances.
We use it since (~Dec 2019) and everything works pretty good.