Filter to display sites by a selected plan

### What

I would like to have the option to display a filter dynamically when there are sites with more than one plan in Calypso on the Sites page: https://wordpress.com/sites.

### Why

The option to filter sites by Plan will be helpful when managing a large number of sites from a single WordPress.com account.

### How

At this time, we have the option to filter sites by **Status**. I guess we can follow a similar approach to have something in place to display filters dynamically when there are sites with more than one plan on the WordPress.com account.