feat(dialog): add ariaLabelledBy config option

[jtcrowson]

This change adds the ability to manually set the aria-labelledby attribute of a MatDialog.

Users can set the aria-labelledby without the mat-dialog-title directive. Users can provide multiple id's.

It is described as an attribute of MatDialogConfig in the Dialog Accessibility Documentation, but is not implemented. This was also mentioned as a follow up to #5176 (comment).

No breaking changes. Existing functionality is preserved without setting the ariaLabelledBy config attribute.

[crisbeto]

LGTM, one nit.

[jtcrowson]

@crisbeto fixed your comment. Anything I need to fix to get the branch-manager passing? I have already rebased.

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "branch-manager" is failing

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[crisbeto]

Nope, that failing CI check will be sorted out by the caretaker.

[jelbourn]

LGTM

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