[MM-54994] Use local requests instead of HTTP requests in the flow library

[hanzei]

Summary

Downstream mattermost/mattermost#26471

Ticket Link

https://mattermost.atlassian.net/browse/MM-54994

[mickmister]

@hanzei Do you think we should defer to release testing for this PR?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 33.35%. Comparing base (d08f7f7) to head (6be57d4).
Report is 2 commits behind head on master.

Files	Patch %	Lines
server/setup_flow.go	0.00%	16 Missing ⚠️
server/plugin.go	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1034      +/-   ##
==========================================
+ Coverage   33.13%   33.35%   +0.22%     
==========================================
  Files          53       53              
  Lines        7983     8001      +18     
==========================================
+ Hits         2645     2669      +24     
+ Misses       5111     5109       -2     
+ Partials      227      223       -4     

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