JWT, KeyAuth, CSRF multivalue extractors

[aldas]

• Adds to JWT, KeyAuth, CSRF support for multivalue extractor - This is mostly useful with headers but this PR adds this other variants also. Usecase - You are using JWT middleware and expect Authentication header with value Bearer xxxx but your intracture has upstream proxy that adds Basic authentication also. Now even if you fill basic auth in browser and your application sends requests with JWT token you would be in trouble as previously JWT middleware knows only to extract first Authentication header value - which could be JWT token but could be also Basic Auth. This change allows extractor to return all those header values and run JWT token checks or validation for Keyauth on them.
• Add flag NoErrorContinuesExecution to JWT and KeyAuth middleware to allow continuing execution next(c) when error handler decides to swallow the error (returns nil). Usecase: This is useful in cases when portion of your site/api is publicly accessible and has extra features for authorized users. In that case you can use ErrorHandlerWithContext to set default public JWT token value to request and continue with handler chain. Note: this is similar to Allow JWT middleware to gracefully fail #2048

p.s. there are ugly error handling parts just to preserve similar/same errors that those middlewares previously returned. Unfortunately all these 3 are quite inconsistent how they do error handling - JWT has 2 generic types. Keyauth has specific error values.

[codecov]

Codecov Report

Merging #2060 (c3edf89) into master (6f6befe) will increase coverage by 0.44%.
The diff coverage is 94.76%.

@@            Coverage Diff             @@
##           master    #2060      +/-   ##
==========================================
+ Coverage   91.57%   92.02%   +0.44%     
==========================================
  Files          33       34       +1     
  Lines        2921     2972      +51     
==========================================
+ Hits         2675     2735      +60     
+ Misses        157      149       -8     
+ Partials       89       88       -1     

Impacted Files	Coverage Δ
echo.go	94.20% <ø> (ø)
middleware/middleware.go	94.44% <ø> (ø)
middleware/csrf.go	88.88% <87.09%> (+8.60%)	⬆️
middleware/key_auth.go	94.91% <92.85%> (-2.01%)	⬇️
middleware/jwt.go	92.77% <93.75%> (+2.50%)	⬆️
middleware/extractor.go	98.83% <98.83%> (ø)

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[aldas]

@lammel if you have time could you review. I like to include it in next release.

[lammel]

@lammel if you have time could you review. I like to include it in next release.

Will look into it today.

[lammel]

Looks good.
When comments from the review are resolved we should merge.

[lammel]

Guess that looks pretty good now. Cool stuff @aldas