fix: panic on json encoding

[gfrancqu]

using json output sometimes trigger a panic, issue seems to happen because a goroutine modify the output object while it is being encoded, adding a mutex solve the issue

[gfrancqu]

fix #800

[codecov[bot]]

Codecov Report

Merging #845 (543bdde) into develop (5f07a52) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #845   +/-   ##
========================================
  Coverage    39.46%   39.46%           
========================================
  Files           42       42           
  Lines         5307     5307           
========================================
  Hits          2094     2094           
  Misses        3115     3115           
  Partials        98       98           

Impacted Files	Coverage Δ
github.com/OWASP/Amass/requests/request.go	62.75% <0.00%> (ø)

[caffix]

The changes have caused the lints/tests to fail and will not be an acceptable solution