Allow priority to be passed as a string

[nienow]

There are two issues surrounding this: #870 and #642, but I've seen mentions of this limitation when trying to use other tools as well. I see there is debate on whether this should be done or not, but I want it for my server, so I was going to implement it anyway.

I'm newer to Go, so I apologize if this could be done simpler, but this was the best way I found.

I created an intermediate struct so I didn't have to change the type of priority on MessageExternal.

[eternal-flame-AD]

Hi, whether this "should be done or not" really needs to be deferred to Jannis but his position is pretty clear and I respect that.

However , You do mention "I've seen mentions of this limitation when trying to use other tools as well", do you know some concrete examples? I think this is the major source of contention so if you have concrete examples I think he might reconsider.

Code wise I think no substantiative change is needed, nitpicks can be deferred later, one potential concern is please verify floating point strings are rejected (NaN, 1.2, 1.2e-3, etc).

Edit: I have to be away for most of next week, consider me green lighted this PR on the condition Jannis is satisfied with the demonstrated need.

[nienow]

As for the tools, home assistant is the big one. There is a workaround by setting up multiple notification services with different default priorities, its just not ideal. Some discussion here: https://community.home-assistant.io/t/self-hosted-push-notifications-with-gotify/120307?page=2

huginn is another: huginn/huginn#2661

And I know I've run into this with at least one other tool in the past, and had found discussions about others having the same issue, but honestly don't remember what it was (and its obviously a tool I don't use anymore).