feature: 添加自定义模板文件路径选项

[slightc]

用户可以自定义修改模板

const { generateService } = require('@umijs/openapi')

generateService({
    requestLibPath: "import { Request as request } from 'umi'",
    schemaPath: 'http://petstore.swagger.io/v2/swagger.json',
    serversPath: './servers',
    templatesFolder: './templates'
})