Support gateway mode for windows installer

[jchengsfx]

• Add gateway_config.yaml to the MSI
• Add --mode <agent|gateway> option to the installer script (defaults to agent)
• Set and use the SPLUNK_CONFIG env var in the registry instead of the --config argument for the collector service
• Update msi/installer test in circleci for agent and gateway modes

[codecov]

Codecov Report

Merging #231 (7a59cde) into main (f5581fe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #231   +/-   ##
=======================================
  Coverage   89.03%   89.03%           
=======================================
  Files          18       18           
  Lines        1021     1021           
=======================================
  Hits          909      909           
  Misses         78       78           
  Partials       34       34           

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