Allow links and tooltips on connections

[sontags]

Currently, links and tooltips are ignored when placed on a connection:

a -> b: connects {
  link: "https://a-b.io/"
}

In my opinion there are a lot of use cases where additional info on a connection can be useful. Rather than adressing the issue by throwing errors (as described in https://github.com/terrastruct/d2/issues/1482) it would be beneficial to see these features being actually implemented.

An example: I use d2 to document systems and their dependencies. A systeme can refere to external system documetation via link. Connections are often a representation of a certain process. I would happily use links on connections to refer to process documentation.