added new resources and datasources

[ValeruS]

Hi, @magne

This PR introduces a new resources and datasources

the list of new datasources:

• user (docs/acctest)
• login (docs/acctest)
• database_schema (docs/acctest)
• database_role (docs/acctest)
• database_permission (docs/acctest)
• database_credential (docs/acctest)
• azure_external_datasource (docs/acctest) is limited to Azure SQL Database only

the list of new resources:

• database_schema (docs/acctest)
• database_role (docs/acctest)
• database_permission (docs/acctest)
• database_masterkey (docs/acctest)
• database_credential (docs/acctest)
• azure_external_datasource (docs/acctest) is limited to Azure SQL Database only

SQL user create functionality has been updated for authType = EXTERNAL

• the second plan show update in place changing role from to

SQL user delete functionality has been updated

• when you try to delete a user and it is the owner of the role/schema, it is not possible until you change the owner (SQL error)

Implemented validation for names/password