Add private endpoint support for mysql flexible server

[chianw]

2048

Address the issue - #2048

PR Checklist

---

• [ x] I have added example(s) inside the [./examples/] folder
• I have added the example(s) to the integration test list for normal (~30 minutes) or long runner >30 minutes
• I have checked the coding conventions as per the wiki
• [x ] I have checked to ensure there aren't other open Pull Requests for the same update/change?

Description

Does this introduce a breaking change

• YES
• [x ] NO

Testing

Refer to https://github.com/chianw/aztfmod-mysql-flexible-server for example of testing this

[arnaudlh]

lgtm