Made ID optional, implemented state tracking

[pnpdev10]

Summary of Fixes in dnacenter/resource_global_credential_v2.go

The main issue was a critical bug in the Create function (lines 777-778) that caused the "Provider produced inconsistent result after apply" error.

Root Cause:

The provider was creating credentials successfully in DNA Center but failing to track them in Terraform state due to an empty resource ID.

resourceMap := make(map[string]string)  // Empty map!
d.SetId(joinResourceID(resourceMap))    // Results in empty/invalid ID

Key Fixes Applied:

• Proper Resource ID Management:
  Now uses the task ID from DNA Center as the resource identifier
  Falls back to timestamp-based ID if no task ID is available

• State Population After Creation:
  Immediately reads back credentials from DNA Center after creation
  Populates Terraform state with the actual credential data

• Better Error Handling:
  Properly checks task completion status
  Reports detailed failure reasons from DNA Center

Impact:

✓ Eliminates "Root object was present, but now absent" errors
✓ Maintains proper state tracking between Terraform runs
✓ Ensures resource consistency between Terraform and DNA Center

The fix ensures that created credentials are properly tracked and managed by Terraform, preventing state drift and enabling reliable resource management.