feat(MatSort): Enable multi-sorting capability on current Interface

### Feature Description

Currently MatSort has a simplistic design that enables sorting only on one column at a time.

Most of enterprise level applications should need multi-sorting capabilities on its datasets. And having it included would be really nice.

In simple terms, my proposal would be to expose a boolean input `matMultiSort` flag and refactor `MatSort` and `MatSortHeader` so that `MatSort` will hold the `MatSortable`'s state, and expose `isActive(id: string) => boolean` and `getNextDirection(id: string) => SortDirection` that will be used instead of directly accessing `active` and `direction`.

`MatSort` will hold a `sortState: Map` that can be used to query its state whenever a sort event occurs.

This change should avoid most (if not all) compatibility issues. But it would be best if we can deprecate the current `sortChanged: EventEmitter` for one that emits `Sort[]` instead, this way we wont need to expose `sortState` (although most of the usage examples for this API expect one to access its internal state anyway).

#### Known issues
This change of sorting behavior will require a review of the included sorting method for `MatTableDataSource`, that was designed around a single column sort.
Having multiple possible sortable columns will require a better strategy on sorting (weighting by index position?).
Or, whenever the MatTableDataSource default implementation is used, multi-sorting could be disabled?

### Use Case

We want to be able to enable sorting based on multiple columns at a time.

This should be done with minimal changes.

Proposed usage:

```html

Name

{{element.name}}

Symbol

{{element.symbol}}

```

```typescript
@ViewChild(MatSort) matSort: MatSort;

ngAfterViewInit() {
matSort.sortChanged.pipe(map(() => Array.from(matSort.sortstate.values()))
.subscribe((sorts: Sort[])=> {
// do something with the Sort[]
});
}
```

Start with the MatSort and MatSortHeader entry points, then review the MatTableDataSource sorting method called out in the issue. Trace how sortChanged, active, and direction currently represent state before assessing the proposed multi-sort API. Done means multi-column sorting is supported without unresolved compatibility or default data-source behavior.

Written by the indexing model from the issue text.