www-form-urlencoded codec

[jeroenrinzema]

This PR introduces a www-form-urlencoded codec which could be used to encode messages.

[codecov-commenter]

Codecov Report

Merging #110 into master will increase coverage by 0.70%.
The diff coverage is 96.41%.

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
+ Coverage   85.57%   86.28%   +0.70%     
==========================================
  Files          82       85       +3     
  Lines        4687     4957     +270     
==========================================
+ Hits         4011     4277     +266     
- Misses        411      413       +2     
- Partials      265      267       +2     

Impacted Files	Coverage Δ
pkg/transport/protocol.go	100.00% <ø> (ø)
pkg/codec/www-form-urlencoded/form.go	92.06% <92.06%> (ø)
pkg/codec/json/object.go	82.40% <100.00%> (ø)
pkg/codec/json/types.go	99.40% <100.00%> (ø)
pkg/codec/www-form-urlencoded/errors.go	100.00% <100.00%> (ø)
pkg/codec/www-form-urlencoded/types.go	100.00% <100.00%> (ø)
pkg/flow/caller.go	69.81% <100.00%> (+0.87%)	⬆️
pkg/references/forwarding/references.go	92.59% <100.00%> (ø)
pkg/references/store.go	100.00% <100.00%> (+1.66%)	⬆️
pkg/transport/http/caller.go	82.30% <100.00%> (+1.76%)	⬆️
... and 5 more