Use one grafana-ntfy connector for multiple server & Priority support

[hugoallaine]

This PR contains code to support forward message to a ntfy server and priority.

Instead of starting the grafana-ntfy connector by giving a ntfy server address, the address is extracted from the url of the request (and priority too if precised). In Grafana, you can write this in the url contact point for example:
http://<grafana-ntfy-connector-addr>:8080?url=http://<ntfy-server-addr>:3500/<topic>&priority=5
It allows also to send to multiple topics of a ntfy server without starting another connector.

This is my first time with Go, please give me tips if something is badly written.

This PR contains the code from @angaz #4 for forward auth (cause I used it for my project)

[academo]

Hi @hugoallaine thanks so much for your PR. I just got some time after holidays to take a look at it.

I merged #4 to main so now this PR only shows your changes.

I like the idea of getting the url from the Grafana request but your current approach is a big breaking change. It'll require anyone using the tool to reconfigure their alerting.

What if instead of removing the option to define the ntfy url as a flag to the program, you modify the sendNotification (and the code that calls it) so if it detects the request comes with the specific ntfy-url it overwrites whatever is defined via flag?

This is an easy way to keep backwards compatibility and still support your use case.