SQL: add MSSQL support and allow storing only a subset of event fields

[creideiki]

This pull request enhances IntelMQ's SQL database support in three ways:

1. Add support for MSSQL servers in the SQLMixin class, requiring the package pymssql.
2. Add a parameter fields to SQLOutputBot, containing a list of which event fields to store in the database. If this is not specified, store all fields as before.
3. Add a parameter reconnect_delay to SQLMixin, making it wait that many seconds before attempting to reconnect after a connection error.
   • We found out the need for this the hard way, when the DB server rebooted and IntelMQ's tight reconnect loop filled the entire disk with error logs before the DB server came back.

[codecov-commenter]

Codecov Report

Merging #2172 (0b45ea6) into develop (80a149a) will decrease coverage by 0.02%.
The diff coverage is 53.57%.

@@             Coverage Diff             @@
##           develop    #2172      +/-   ##
===========================================
- Coverage    76.33%   76.31%   -0.03%     
===========================================
  Files          441      441              
  Lines        23686    23710      +24     
  Branches      3747     3753       +6     
===========================================
+ Hits         18081    18094      +13     
- Misses        4865     4876      +11     
  Partials       740      740              

Impacted Files	Coverage Δ
intelmq/lib/mixins/sql.py	51.25% <25.00%> (-5.68%)	⬇️
intelmq/bots/outputs/sql/output.py	94.28% <91.66%> (-1.87%)	⬇️
intelmq/bots/experts/reverse_dns/expert.py	83.87% <0.00%> (+1.61%)	⬆️

[sebix]

Can't test it, but looks good overall.

Entry in changelog is missing

[sebix]

Suggested change

	fields = None
	fields: Optional[list] = None