validator: add support for protoc-gen-validate 0.6.0

[danielhochman]

bufbuild/protoc-gen-validate#455 changed the interface for the Validate call from Validate() to Validate(bool).

I modified the middleware to support both.

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[codecov-commenter]

Codecov Report

Merging #417 (a703562) into master (165f605) will increase coverage by 0.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #417      +/-   ##
==========================================
+ Coverage   73.98%   74.37%   +0.38%     
==========================================
  Files          42       42              
  Lines        1357     1362       +5     
==========================================
+ Hits         1004     1013       +9     
+ Misses        299      295       -4     
  Partials       54       54              

Impacted Files	Coverage Δ
validator/validator.go	91.66% <100.00%> (+2.19%)	⬆️
retry/retry.go	75.92% <0.00%> (+2.46%)	⬆️

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[johanbrandhorst]

LGTM

[johanbrandhorst]

Thanks for your contribution! Could you please adapt this for our v2 branch? Thanks!