Opsgenie URL hardcoded "ApiUrl: api.opsgenie.com". Not possible to change to "api.eu.opsgenie.com"

## Description

## What did you expect to happen?
To be able to add in the config file the URL, something like:
url: https://api.eu.opsgenie.com

## What happened instead?

## Output of run with `POSTEE_DEBUG=true`:
postee INFO[2022-11-09T15:42:34.961970252Z] Client is configured with ApiUrl: api.opsgenie.com, LogLevel: info, RetryMaxCount: 4
```
(paste your output here)
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## Additional details (environment setup, networking info...):

Start by locating the Opsgenie client configuration and the hardcoded ApiUrl value, then inspect how configuration values are parsed. Done means the Opsgenie URL can be set to api.eu.opsgenie.com through the configuration and the resulting client uses that value.

Written by the indexing model from the issue text.