Support expression in additional appeals criteria

[Rahmat Hidayat]

Summary
Additional appeals are created when the specified criteria are fulfilled. Currently, the criteria only support a few fields around resource. Therefore, it's not possible to create criteria based on other appeal properties such as account_id or account_type.

Proposed solution
Introduce expression in the RequirementTrigger and evaluate using existing expression library. Available parameter would be $appeal
Additionally, once we have expression, existing Conditions won't be necessary as expression will cover anything support by Conditions right now. We can deprecate or even delete it.

type RequirementTrigger struct {
	ProviderType string
	ProviderURN  string
	ResourceType string
	ResourceURN  string
	Role         string
- 	Conditions   []*Condition
+	Expression   string 
}