Relax MatTableDataSource’s filter to be of type “{}”

#### Feature Description
Relax `MatTableDataSource.filter` to accept `{}`. Right now, it is restricted to `string`.
This would allow *much* easier custom filtering on multiple dimensions.

#### Use Case
When the filter is “only” a string, it makes it very difficult to “structurally” filter a table on multiple dimensions, even when customizing `filterPredicate` . Some recommendations on internet include:
- passing a JSon object, and parsing the JSon `filterPredicate`, for each and every row: http://nataliesmith.ca/blog/angular-material-filter-table
- everything in this bug thread: https://github.com/angular/components/issues/6178

While it works, it’s also a gratuitous workaround: It turns out that at no point does `MatTableDataSource` actually *require* filter to be a `string`: Whatever gets passed to `filter` just gets piped “as-is” to `filterPredicate`.

Here is a simple fork of “Table with filtering” that filters on different columns individually, where all I did was pass my complex object, while not respecting :
https://stackblitz.com/edit/table-filtering-multiple-filters-example?file=app/table-filtering-example.html
https://table-filtering-multiple-filters-example.stackblitz.io/

#### Caveats

It would still be up to the caller to make sure that their `filter` object is homogeneous with their `filterPredicate` object.

Start at the MatTableDataSource.filter entry point and inspect how its value is passed to filterPredicate. Update the public type to accept {} while preserving the existing predicate flow, then verify that structured multi-dimension filter objects work without requiring JSON conversion.

Written by the indexing model from the issue text.