refactor: gateway

[atzoum]

Description

Doing some preliminary work for being able to perform another refactoring that is going to modify gateway's functions to start relying on sourceID rather than writeKey:

• Stop using global (config) variables in the gateway package
• Stop using global config, logger & stats variables in gateway.Handle
• Organise gateway.Handle fields into logical groups
• Adapt gateway#Setup to fill in necessary configuration moved from global configuration
• Remove methods/variables that are no longer necessary
• Split gateway.go into more files containing logically grouped gateway functionalities
• Wait for gateway to initialise its backend configuration in gateway's tests

Linear Ticket

Link

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Patch coverage: 80.08% and project coverage change: +0.11% 🎉

Comparison is base (7719719) 68.17% compared to head (bf88c80) 68.29%.

❗ Current head bf88c80 differs from pull request most recent head 26499ca. Consider uploading reports for the commit 26499ca to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3708      +/-   ##
==========================================
+ Coverage   68.17%   68.29%   +0.11%     
==========================================
  Files         320      324       +4     
  Lines       50469    50437      -32     
==========================================
+ Hits        34409    34446      +37     
+ Misses      13836    13765      -71     
- Partials     2224     2226       +2     

Files Changed	Coverage Δ
runner/runner.go	70.28% <ø> (-0.12%)	⬇️
app/apphandlers/gatewayAppHandler.go	13.33% <33.33%> (+0.71%)	⬆️
gateway/handle_http.go	59.18% <59.18%> (ø)
app/apphandlers/embeddedAppHandler.go	76.22% <66.66%> (+0.18%)	⬆️
gateway/handle.go	79.74% <79.74%> (ø)
gateway/handle_lifecycle.go	85.71% <85.71%> (ø)
gateway/handle_config.go	98.03% <98.03%> (ø)
gateway/import_handler.go	100.00% <100.00%> (ø)
gateway/regular_handler.go	100.00% <100.00%> (ø)

... and 16 files with indirect coverage changes

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[lvrach]

Most of the comments are irrelevant for this PR since large portions of code moved and this is preparatory PR.

[atzoum]

Most of the comments are irrelevant for this PR since large portions of code moved and this is preparatory PR.

tried to address them nevertheless :)