Add new output module for Splunk

[w0Tx]

This PR include a new way to export data to a splunk instance through HTTP Event Collector.

I've decided to create a new output module instead of modifying the HTTP output module to keep the code as simple as possible.

I've also added the documentation associated with it.

[TheTechromancer]

Thanks for doing this! The module looks solid, we just need to write a test for it.

[codecov]

Codecov Report

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

Comparison is base (25e5390) 92% compared to head (6967bce) 92%.
Report is 8 commits behind head on dev.

Files	Patch %	Lines
bbot/modules/output/splunk.py	84%	5 Missing ⚠️
...est/test_step_2/module_tests/test_module_splunk.py	93%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##             dev   #1091   +/-   ##
=====================================
- Coverage     92%     92%   -0%     
=====================================
  Files        306     308    +2     
  Lines      19396   19468   +72     
=====================================
+ Hits       17805   17863   +58     
- Misses      1591    1605   +14     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[TheTechromancer]

Tests written and passing, waiting for approval.