feat: /status endpoint supports weighted choice

[mccutchen]

Fixes compatibility with the original httpbin by making the /status endpoint accept multiple, optionally weighted status codes to choose from. Per the description in #145, this implementation attempts to match original httpbin's behavior:

• If not specified, weight is 1
• If specified, weights are parsed as floats, but there is no requirement that they sum to 1.0 or are otherwise limited to any particular range

Fixes #145.

[codecov]

Codecov Report

Merging #162 (501020e) into main (1a41486) will increase coverage by 0.07%.
The diff coverage is 98.07%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
+ Coverage   95.04%   95.11%   +0.07%     
==========================================
  Files           9        9              
  Lines        2119     2171      +52     
==========================================
+ Hits         2014     2065      +51     
- Misses         71       72       +1     
  Partials       34       34              

Files	Coverage Δ
httpbin/handlers.go	99.57% <100.00%> (+<0.01%)	⬆️
httpbin/helpers.go	96.74% <97.29%> (+0.06%)	⬆️