Describe the bug

AxiosRequestHeaders is currently only a Record for primitives Record<string, string | number | boolean> and, according to typing cannot have object as values. In reality, headers under keys can objects with other keys. A good example is from README:

axios.defaults.headers.common['Authorization'] = AUTH_TOKEN;

axios.defaults.headers.post['Content-Type'] = 'application/x-www-form-urlencoded';

To Reproduce

F.e. if we were to delete a header entry in transformRequest we would get a typescript error Property 'Authorization' does not exist on type 'string'.

 return axios({
        method : 'get',
        transformRequest: (data, headers) => {
            delete headers?.common?.Authorization;
        }
    })

Expected behavior

To be able to access / delete headers under the whole list of keys common, post, etc

Environment

• Axios Version 0.24.0
• Adapter HTTP

Additional context/Screenshots

This is a typescript bug