COS module needs updates around monitoring and activity tracking for buckets

[ocofaigh]

Some changes are coming in IBM-Cloud/terraform-provider-ibm#5277 which will require some refactoring of the module...

Current code works like this:

• When values are passed for activity_tracker_crn and sysdig_crn, this is what determines if activity tracking or monitoring should be enabled for buckets.

Refactoring considerations:

• activity_tracker_crn and metrics_monitoring_crn are now becoming optional by the provider. If they are not specified, it will default to the default AT or Sysdig instances in the account. So with this in mind, we should probably create new variables enable_metrics_monitoring and enable_activity_tracking which default to true (see how upgrade test handles this).
• management_events is also coming in new provider drop for AT, so that should be set to true as well.
• take this opportunely to rename sysdig_crn to monitoring_crn

NB: modules/fscloud, modules/buckets, solutions/secure-cross-regional-bucket, solutions/secure-regional-bucket will all need updates based on the refactor.