Enhance the mat-paginator so the total length can be masked.

#### Feature Description

Provide the way to mask the 'total length'.

#### Use Case

With large data set, getting the count from the backend database is slow. We'd like to mask the "total length", so the users can paginate without having to know the max length. The end user would be able to continue to paginate until there is no results from the backend.

Two issues

1. You wouldn't know the actual max length.
2. If the last page set returns the same as the page size, then it would still allow to paginate to next page. However, there would be no results.

The end user who can live with these two issues can simply mask the total length by specifying the display label such as "More" or "..."

Example.

200-300 (More) "<" ">"
Or
200-300 (...) "<" ">"

Start with the mat-paginator entry point and inspect its current total-length, range-label, and next-page behavior. Define how maskTotal and maskTotalLabel should affect the displayed range and pagination, including the two stated last-page cases. Done means the behavior is specified and covered for masked totals and empty-result navigation.

索引モデルが issue の本文から書いたものです。