Expose authorizing context in Daml

[cocreature]

I often find myself needing access to the parties that are currently authorizing my update block. I can always get access by looking at the controllers & signatories & recomputing it but this is somewhat annoying & error prone especially given #13793 which means I have to repeat the full controller expression. A primitive authorizingParties : Update [Party] would be nice for this.

[basvangijzel-DA]

@cocreature Are you still running into this issue? We're happy to have a look at it if so.

[cocreature]

I still find a use for this occasionally but I’d say it’s low priority. Let’s get upgrading working first :)