http: explicitly set and use http client in outbound

[jronak]

HTTP outbound now uses the http.Client explicitly set on the outbound struct than reading the client from yarpc HTTP transport. This is a minor cleanup done which is used in the HTTP outbound TLS integration where each outbound gets a separate HTTP client to do mTLS.

[codecov[bot]]

Codecov Report

Base: 85.23% // Head: 85.25% // Increases project coverage by +0.02% :tada:

Coverage data is based on head (d7b9182) compared to base (cca6727). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2175      +/-   ##
==========================================
+ Coverage   85.23%   85.25%   +0.02%     
==========================================
  Files         269      269              
  Lines       15352    15353       +1     
==========================================
+ Hits        13085    13089       +4     
+ Misses       1846     1844       -2     
+ Partials      421      420       -1     

Impacted Files	Coverage Δ
transport/http/outbound.go	92.09% <100.00%> (+0.02%)	:arrow_up:
transport/internal/tls/dialer/dialer.go	100.00% <0.00%> (+8.82%)	:arrow_up:

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