Adds controls for impersonation requests.

Fixes #5352

```yaml
allow:
  impersonate:
    users: ['alice', 'bob']
    roles: ['*']
    where: 'contains(user.spec.traits["groups"], impersonate_role.traits)'
```

Adds "impersonator" to all X.509 and SSH client certs
issued using impersonation and does best effort to track
requests by impersonators in audit events.

Limits certs TTL to the impersonator's cert TTL.
Prevents impersonating users to recursively impersonate
other users.
Allows impersonating users to renew their own certificate,
for example to set route to cluster.

Adds missing token permission for editor role.

api/types/events/events.proto

@@ -44,6 +44,9 @@ message UserMetadata {
 
     // Login is OS login
     string Login = 2 [ (gogoproto.jsontag) = "login,omitempty" ];
+
+    // Impersonator is a user acting on behalf of another user
+    string Impersonator = 3 [ (gogoproto.jsontag) = "impersonator,omitempty" ];
 }
 
 // Server is a server metadata

api/types/role.go

@@ -111,6 +111,11 @@ type Role interface {
 	GetDatabaseUsers(RoleConditionType) []string
 	// SetDatabaseUsers sets a list of database users this role is allowed or denied access to.
 	SetDatabaseUsers(RoleConditionType, []string)
+
+	// GetImpersonateConditions returns conditions this role is allowed or denied to impersonate.
+	GetImpersonateConditions(rct RoleConditionType) ImpersonateConditions
+	// SetImpersonateConditions returns conditions this role is allowed or denied to impersonate.
+	SetImpersonateConditions(rct RoleConditionType, cond ImpersonateConditions)
 }
 
 // NewRole constructs new standard role
@@ -179,6 +184,9 @@ func (r *RoleV3) Equals(other Role) bool {
 		if !r.GetKubernetesLabels(condition).Equals(other.GetKubernetesLabels(condition)) {
 			return false
 		}
+		if !r.GetImpersonateConditions(condition).Equals(other.GetImpersonateConditions(condition)) {
+			return false
+		}
 	}
 
 	return true
@@ -472,6 +480,27 @@ func (r *RoleV3) SetDatabaseUsers(rct RoleConditionType, values []string) {
 	}
 }
 
+// GetImpersonateConditions returns conditions this role is allowed or denied to impersonate.
+func (r *RoleV3) GetImpersonateConditions(rct RoleConditionType) ImpersonateConditions {
+	cond := r.Spec.Deny.Impersonate
+	if rct == Allow {
+		cond = r.Spec.Allow.Impersonate
+	}
+	if cond == nil {
+		return ImpersonateConditions{}
+	}
+	return *cond
+}
+
+// SetImpersonateConditions returns conditions this role is allowed or denied to impersonate.
+func (r *RoleV3) SetImpersonateConditions(rct RoleConditionType, cond ImpersonateConditions) {
+	if rct == Allow {
+		r.Spec.Allow.Impersonate = &cond
+	} else {
+		r.Spec.Deny.Impersonate = &cond
+	}
+}
+
 // GetRules gets all allow or deny rules.
 func (r *RoleV3) GetRules(rct RoleConditionType) []Rule {
 	if rct == Allow {
@@ -592,7 +621,19 @@ func (r *RoleV3) CheckAndSetDefaults() error {
 			return trace.BadParameter("failed to process 'deny' rule %v: %v", i, err)
 		}
 	}
-
+	if r.Spec.Allow.Impersonate != nil {
+		if err := r.Spec.Allow.Impersonate.CheckAndSetDefaults(); err != nil {
+			return trace.Wrap(err)
+		}
+	}
+	if r.Spec.Deny.Impersonate != nil {
+		if r.Spec.Deny.Impersonate.Where != "" {
+			return trace.BadParameter("'where' is not supported in deny.impersonate conditions")
+		}
+		if err := r.Spec.Deny.Impersonate.CheckAndSetDefaults(); err != nil {
+			return trace.Wrap(err)
+		}
+	}
 	return nil
 }
 
@@ -651,6 +692,37 @@ func (r *RoleConditions) Equals(o RoleConditions) bool {
 	return true
 }
 
+// IsEmpty returns true if conditions are unspecified
+func (i ImpersonateConditions) IsEmpty() bool {
+	return len(i.Users) == 0 || len(i.Roles) == 0
+}
+
+// Equals returns true if the impersonate conditions (logins, roles
+// and rules) are equal and false if they are not.
+func (i ImpersonateConditions) Equals(o ImpersonateConditions) bool {
+	if !utils.StringSlicesEqual(i.Users, o.Users) {
+		return false
+	}
+	if !utils.StringSlicesEqual(i.Roles, o.Roles) {
+		return false
+	}
+	if i.Where != o.Where {
+		return false
+	}
+	return true
+}
+
+// CheckAndSetDefaults checks and sets default values
+func (i ImpersonateConditions) CheckAndSetDefaults() error {
+	if len(i.Users) != 0 && len(i.Roles) == 0 {
+		return trace.BadParameter("please set both impersonate.users and impersonate.roles")
+	}
+	if len(i.Users) == 0 && len(i.Roles) != 0 {
+		return trace.BadParameter("please set both impersonate.users and impersonate.roles")
+	}
+	return nil
+}
+
 // NewRule creates a rule based on a resource name and a list of verbs
 func NewRule(resource string, verbs []string) Rule {
 	return Rule{