feat(opsgenie): Add support for setting alias in Opsgenie notification

[gazidizdaroglu]

This PR adds support for setting the alias field in the Opsgenie notifications.

[codecov[bot]]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (4222fe0) 54.99% compared to head (0803b19) 55.14%.

Files	Patch %	Lines
pkg/services/opsgenie.go	42.85%	7 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #268 +/- ## ========================================== + Coverage 54.99% 55.14% +0.15% ========================================== Files 35 35 Lines 3386 3400 +14 ========================================== + Hits 1862 1875 +13 Misses 1251 1251 - Partials 273 274 +1 ```

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[pasha-codefresh]

@gazidizdaroglu Great job, could you please add this parameter description to documentation ?

[gazidizdaroglu]

@pasha-codefresh sure, let me update the documentation

[gazidizdaroglu]

@pasha-codefresh I've updated the doc

[pasha-codefresh]

Wow, amazing work, thank you