[crystal] implement "multi" params

See: https://github.com/mamantoha/crest?tab=readme-ov-file#crestnestedparamsencoder

Author: n-rodriguez

modules/openapi-generator/src/main/resources/crystal/api.mustache

@@ -131,7 +131,7 @@ module {{moduleName}}
       cookie_params = Hash(String, String).new
 
       # query parameters
-      query_params = Hash(String, String).new
+      query_params = Hash(String, String | Array(String)).new
       {{#queryParams}}
       query_params["{{{baseName}}}"] = {{#collectionFormat}}@api_client.build_collection_param({{{paramName}}}, :{{{collectionFormat}}}) unless {{{paramName}}}.nil?{{/collectionFormat}}{{^collectionFormat}}{{{paramName}}}.to_s unless {{{paramName}}}.nil?{{/collectionFormat}}
       {{/queryParams}}

modules/openapi-generator/src/main/resources/crystal/api_client.mustache

@@ -109,8 +109,7 @@ module {{moduleName}}
       when :pipes
         param.join("|")
       when :multi
-        # TODO: Need to fix this
-        raise "multi is not supported yet"
+        param
       else
         raise "unknown collection format: #{collection_format.inspect}"
       end
@@ -147,7 +146,8 @@ module {{moduleName}}
         cookies: cookie_params,
         form: form_or_body,
         logging: @config.debugging,
-        handle_errors: false
+        handle_errors: false,
+        params_encoder: Crest::NestedParamsEncoder
       )
 
       response = request.execute