Add Postgres/MySql/ManagedInstance online migration scenarios

[ash1625]

Description

REST spec PR link to the SDK PR :
Azure/azure-rest-api-specs#4627

This PR adds the below new scenarios to Azure Database Migration Service:

1. SQL to Azure SQL Managed Instance online migration
2. SQL to Azure Postgres SQL online migration
3. SQL to Azure MySQL online migration

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[dsgouda]

Looks good for the most part, needs a minor change,

[dsgouda]

LGTM