[Feature Request]: add option to assign labels/titles of each filter

### Feature description

I had asked @donyunardi about how to control the label that appears with filter variables in an app. He explained that the label comes from the attribute of label for the specific column `attr(data[[x]], "label")` and no descriptive is shown when label is null.

Could the `title` input parameter of `teal_slice` be used to control the descriptive information for filter variables? This would be a straightforward way to implement more customized filters with clear meanings for end-users of the deployed app.

Thank you for considering this!

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