Add ability to enable a managed identity to have access to specified SQL database

[jananivMS]

Closes #966

This is a customer ask to enable the specified managed identity as a DB user for the specified database.

The assumption here based on the customer's suggestion is that the SQL server specified will already have configured as the AAD admin, the managed identity that is used to run the operator as.

What this PR does / why we need it:

• Creates a new CRD and controller for the new AzureSQLManagedUser
• Logs to the database using the managed identity that the operator is running as
• Executes the command to enable specified managed identity as the DB user

Special notes for your reviewer:

1. Will need to deploy the operator using managed identity IM1)
2. Configure this managed identity (M1) as the AAD admin for the SQL server
3. Then create another managed identity (M2) and specify this identity's name and object ID in the YAML to enable as db user
4. The only way I was able to test this was to redeploy operator as M2 and then try to enable some other Managed user. If the operator is able to successfully login and connect to DB it means the user was successfully enabled in step 3
5. Notice secrets created

How does this PR make you feel:

If applicable:

• this PR contains documentation
• this PR contains tests

[frodopwns]

one change worth making at least

Dismissing as I am re-requesting a review

[frodopwns]

lgtm