Hive alerter update

[ferozsalam]

The organisation where I work makes extensive use of the integration between TheHive and ElastAlert, and we have noticed several issues with the current alerter. These are:

• Undocumented functionality, such as the functionality around customFields
• Unexpected behaviour with aggregations (Aggregation issues with TheHive alerting Yelp/elastalert#2560)
• The entire functionality of the alerter being defined in a single function, making it hard to reason about
• The use of behaviour that is not standard to ElastAlert, such as the syntax for populating the alert_text
• The use of older Python styles (although this applies to other alerters as well)

I have refactored the alerter extensively and am opening this PR to propose integrating the refactored alerter upstream.

As part of this change I have:

• Converted the alerter to use newer Python functionality, such as f-strings and type hints
• Documented the various features that were previously missed, and explained their working
• Refactored the monolithic alert function into several smaller functions that are easier to reason about
• Converted the alerter to use behaviour standard to other ElastAlert alerters, including for aggregated alerts (which should resolve Aggregation issues with TheHive alerting Yelp/elastalert#2560)

This is a breaking change, and some TheHive rules already written will need (minor) refactoring to continue working. Wherever possible, I have attempted to minimise such changes. I do believe that the refactored alerter will be easier to work with over the long-term than the previous version.

The refactored alerter has been tested in a local environment against multiple rule types.

[nsano-rururu]

HiveAlerter doesn't have any test code, so I think you need to add it to tests/alerts_test.py.

[ferozsalam]

@nsano-rururu I have added a test that covers the main functionality of the alerter.

[jertel]

Thanks!