Explicitly passing Rack related configurations

[st0012]

Since RequestInterface is directly initialized from Event, it's entirely possible to pass everything it needs with method calls.

This change will make RequestInterface's dependencies clearer and shouldn't break any user program (interfaces shouldn't be initialized manually).

[codecov-commenter]

Codecov Report

Merging #1662 (10ab72e) into master (d583528) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1662   +/-   ##
=======================================
  Coverage   98.47%   98.47%           
=======================================
  Files         136      136           
  Lines        7655     7656    +1     
=======================================
+ Hits         7538     7539    +1     
  Misses        117      117           

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/event.rb	98.87% <100.00%> (+0.01%)	⬆️
sentry-ruby/lib/sentry/interfaces/request.rb	95.16% <100.00%> (ø)
...y/spec/sentry/interfaces/request_interface_spec.rb	99.01% <100.00%> (ø)

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