Support "tel:" links in webhook cards
- Setting a "tel:0123456789" Url in a MessageCard/potentialAction/[@type=OpenUri] causes the potential action to be skipped from rendering (no button at all)
- If setting a markdown or plain HTML link to a "tel:"-schema URI, then an
hrefattribute is rendered.
See also https://github.com/MicrosoftDocs/AdaptiveCards/issues/216