feat: more options for UI middleware

[fredbi]

• refactored UI middleware

  • factorized UI middleware to remove duplicated code
  • factorized UI middleware options: to avoid breaking changes in the options types, there is a decode/encode to a common structure
  • added more options:
    • allows to fully customize the UI template
  • added more unit tests

• Spec middleware: added support for optional SpecOption argument

  • allows to serve the spec from a custom path / document name

• serving with or without trailing "/" (cf. issue Swagger Middleware drops trailing slash and can't find path #238)

  • replaced path.Join() by path.Clean(), which is the intended behavior (i.e. serve the path, irrespective of the presence of a trailing slash)
  • generalized this behavior to all UI and Spec middleware, not just swaggerUI

• API Context:

  • exposed middleware to serve RapiDoc UI
  • allowed new UIOption (...UIOption) to the APIHandler, etc middleware
  • coordinated UI / Spec middleware to be consistent when non-default path/document URL is served

• fixes add the possibilty to use a custom swagger ui template #192
• fixes ReDoc uses unmodifiable spec path #226

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (248b38c) 80.83% compared to head (f3145ae) 82.04%.

Files	Patch %	Lines
middleware/ui_options.go	85.00%	8 Missing and 4 partials ⚠️
middleware/context.go	95.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
+ Coverage   80.83%   82.04%   +1.20%     
==========================================
  Files          44       45       +1     
  Lines        3366     3420      +54     
==========================================
+ Hits         2721     2806      +85     
+ Misses        535      501      -34     
- Partials      110      113       +3     

Flag	Coverage Δ
oldstable	82.04% <92.89%> (+1.20%)	⬆️
stable	82.04% <92.89%> (+1.20%)	⬆️

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