Add new board_connect_url()

[juliasilge]

Related to rstudio/pins-r#706

In that PR, we are adding a new board for using Connect vanity URLs. This is a read-only board without versioning, so basically like board_url() with the Connect-specific authentication. Most of the work/thought was getting the testing to run well FWIW.

[hathawayj]

Is there an update on when this functionality will be available in the Python package?

[isabelizimm]

Ah, great question @hathawayj -- there isn't a current date for when this will be implemented, but I am happy to prioritize this work if people are waiting on it!