Missing indeterminate state for Progress or a Spinner component

[gdgcentrica]

This function solves the problem (这个功能解决的问题)

It will be really useful to have an "indeterminate" state for the Progress component or a Spinner component, because it's not always possible to have a progress value during some long processing.

Expected API (期望的 API)

<n-progress type="circle" :indeterminate="true" />