soarca-initialized variable properties overwrite playbook variable properties

[lucamrgs]

Describe the bug
Noticeable in trigger/id API calls. Payload-provided variables unspecified fields are initialised to default values, and variables are then completely substituting the playbook variables (playbook.PlaybookVariables.Merge(payload_variables) at the end of func MergeVariablesInPlaybook(playbook *cacao.Playbook, body []byte) (bool, string), in trigger_api.go).

Hence, variables inside the playbook will have their properties changed.

To Reproduce provide details logs and steps
To reproduce, you can add a print before trigger.execute(&playbook, context) in func (trigger *TriggerApi) ExecuteById(context *gin.Context) in trigger_api.go, and run the test func TestExecutionOfPlaybookByIdWithPayloadValidVariables(t *testing.T) in unittest/routes.trigger_api/trigger_api_test.go

Expected behavior
The variable in the playbook remains exactly the same except for the values provided by the properties in the payload-specified variables.

Additional context
As solution, we should implement a function that receiving a variable in input from a payload (not marshalled to a cacao.Variable object), marshals it in a cacao.Variable object but also taking into account the variable it will substitute the values of. This will also allow us to e.g. prevent payload-provided variables from overwriting the "external" property.

Then, we should also implement a property-by-property merge function for the cacao.Variables which accounts for initialisation-provided values, diffs between payload-provided and playbook-present variables, and prevents overwriting of meta properties e.g. bool, and const (modifying these fields should pertain to playbook editing tasks. It's surely not a within-execution task).

[lucamrgs]

This is solved in #206