Feature request - default timeout as provider config

[pregress]

Change the defaultTimeout to be variable that can be set in the provider:

terraform-provider-mssql/mssql/provider.go

Line 27 in b5224f9

defaultTimeout = schema.DefaultTimeout(30 * time.Second)

Why?
If you use azuread_default_chain_auth sometimes you receive a timeout if you depend on Azure CLI credential.

Example usage:

provider "mssql" {
  debug = "false"
  defaultTimeout = 60
}

[PeteAtWSA]

This is also useful for azure sql database with a low performance.
There you get a "db connection failed after 30s timeout" failure frequently.