Using `oobUrl` in `didcomm://` deeplink for linking with shortened url

When using the `didcomm://` scheme to link to a didcomm enabled application, you lose the ability to use shortened urls, as you don't know where to resolve the invitation from.

We've encountered with larger OOB invitations that they exceed the character limit iOS allows and thus it's not possible to use a didcomm deeplink for this purpose.

For this reason we've started using a custom `oobUrl` property on the `didcomm://` deeplink that contains a shortened url.

a working link looks like this:
```
didcomm://?oobUrl=https%3A%2F%2Fparadym.id%2Finvitation%2Fefaf249a-39e5-42ab-9083-0aab96e7c3f1
```

I think it could be useful to standardize this custom parameter for passing a shortened oob url to a didcomm deeplink for the case described above.

Thoughts?

cc @tommylans