refactor HCL provider

[Alma-media]

Related to #140

• use embedded types
• remove code duplicates

Note: since hcl allows having only one ",remain" tag per struct we cannot embed more than one struct

[codecov-io]

Codecov Report

Merging #165 into master will increase coverage by 0.10%.
The diff coverage is 79.48%.

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
+ Coverage   82.23%   82.34%   +0.10%     
==========================================
  Files         144      147       +3     
  Lines        6385     6417      +32     
==========================================
+ Hits         5251     5284      +33     
  Misses        761      761              
+ Partials      373      372       -1     

Impacted Files	Coverage Δ
pkg/transport/graphql/graphql.go	66.01% <0.00%> (-0.65%)	⬇️
pkg/codec/json/json.go	75.67% <50.00%> (+0.67%)	⬆️
pkg/providers/hcl/specs.go	74.67% <52.38%> (+0.06%)	⬆️
pkg/transport/http/caller.go	75.22% <55.55%> (-2.56%)	⬇️
pkg/providers/hcl/parameter_map.go	78.18% <78.18%> (ø)
pkg/providers/schemas.go	73.82% <86.66%> (ø)
pkg/references/referenced.go	91.52% <91.52%> (ø)
pkg/generators/openapi3/generator.go	91.36% <100.00%> (+0.52%)	⬆️
pkg/generators/openapi3/options.go	100.00% <100.00%> (ø)
pkg/transport/http/http.go	33.33% <100.00%> (-24.25%)	⬇️
... and 4 more