feat(components/sort): add multi-sort support

[imerljak]

Adds multi-column sorting capability to MatSort, allowing to sort a table on multiple of its columns at once by toggling matSortMultiple.

This feature adds a new sortState variable inside MatSort that should be used as a source of truth when matSortMultiple is enabled. Fixes #24102

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[ricovivid]

@imerljak when in multi-sort will this also persist the visibility of the sort arrows when multiple columns are sorted?

[imerljak]

@imerljak when in multi-sort will this also persist the visibility of the sort arrows when multiple columns are sorted?

You mean the visual feedback at the table header? Yep. If two cols are sorted they will render the header accordingly.

[dsab123]

Just poking in to say that this is a great idea 👌 Thank you @imerljak and angular team!