Add disabled parameter to provider

[revmischa]

Adds a disabled boolean parameter to the provider configuration that prevents database connections when enabled.

This is needed in my case where I create a database conditionally, and in the case the DB is not created, I don't want this provider to run at all.

When disabled=true, the provider skips connection setup and all operations.

Example usage:

provider "postgresql" {
  disabled = true
}

provider "postgresql" {
  alias = "active"

  scheme    = "awspostgres"
  host      = module.db[0].cluster_endpoint
  port      = module.db[0].port
  database  = module.db[0].database_name
  username  = "postgres"
  password  = module.db[0].postgres_master_password
}