[api] add Connector card

Link unfurling responses supports Adaptive, Hero, Thumbnail, and Connector cards

This is Teams-specific and wasn't in the BF SDK

[Connector card for Microsoft 365 Groups](https://learn.microsoft.com/en-us/microsoftteams/platform/task-modules-and-cards/cards/cards-reference#connector-card-for-microsoft-365-groups)

Start by locating the existing link-unfurling support for Adaptive, Hero, and Thumbnail cards, then compare the required Connector card shape with the linked Microsoft Teams documentation. Implement Connector card support alongside those existing responses and verify that link unfurling can return it. No specific file or test is named in the issue.

Written by the indexing model from the issue text.