feat(context): add ValidateCtx

[shblue21]

This adds c.ValidateCtx(v) to pass the request context to the validator, as discussed in #2399. If the validator doesn't implement ValidateCtx, it calls Validate instead.

I left c.Validate(v) unchanged. Making it call ValidateCtx automatically could skip a wrapper's own validation when it embeds another validator with that method.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.80%. Comparing base (dcb05f0) to head (755c804).
⚠️ Report is 31 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3152      +/-   ##
==========================================
+ Coverage   93.34%   95.80%   +2.45%     
==========================================
  Files          43       46       +3     
  Lines        4735     4244     -491     
==========================================
- Hits         4420     4066     -354     
+ Misses        192      178      -14     
+ Partials      123        0     -123     

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