Masked should be Boolean type

As documented in
https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/api/project_level_variables.md#create-variable
and it's a boolean in `db/schema.rb`

changelogs/unreleased/variables-boolean-type.yml

+---
+title: 'API: change masked attribute type to Boolean'
+merge_request: 28758
+author:
+type: other

lib/api/variables.rb

@@ -55,7 +55,7 @@ module API
         requires :key, type: String, desc: 'The key of the variable'
         requires :value, type: String, desc: 'The value of the variable'
         optional :protected, type: String, desc: 'Whether the variable is protected'
-        optional :masked, type: String, desc: 'Whether the variable is masked'
+        optional :masked, type: Boolean, desc: 'Whether the variable is masked'
         optional :variable_type, type: String, values: Ci::Variable.variable_types.keys, desc: 'The type of variable, must be one of env_var or file. Defaults to env_var'
 
         if Gitlab.ee?
@@ -82,7 +82,7 @@ module API
         optional :key, type: String, desc: 'The key of the variable'
         optional :value, type: String, desc: 'The value of the variable'
         optional :protected, type: String, desc: 'Whether the variable is protected'
-        optional :masked, type: String, desc: 'Whether the variable is masked'
+        optional :masked, type: Boolean, desc: 'Whether the variable is masked'
         optional :variable_type, type: String, values: Ci::Variable.variable_types.keys, desc: 'The type of variable, must be one of env_var or file'
 
         if Gitlab.ee?