[terraform-ibm-cloud-monitoring] Changed needed to sysdig resource key creation

[ocofaigh]

Problem:
In https://github.com/terraform-ibm-modules/terraform-ibm-observability-instances/blob/main/modules/cloud_monitoring/main.tf a resource key is always created. Consumers are reporting it is hard to follow secret rotation best practises because:

• 1 key is always created - even if its not needed
• You currently cannot rotate the key value using terraform. Simply changing the value of manager_key_name will not rotate the key.
• You really need the ability to have 2 active keys at any given time to be able to rotate key consumers (e.g. sysdig agents) non disruptively

Proposal:

• Remove the manager_key_name input and replace it with a new input resource_keys (copy this approach)

Considerations:

• Should the default value of resource_keys be a way where it will create the manager key by default? Or it should create no key by default?
• This will be a breaking change that will impact consumers. How should we handle?