Add FORMAT_CHECKER attribute for Validator

[TiborVoelcker]

Implement an FORMAT_CHECKER attribute for the Validator class. This allows to use the format_checker of an automatically detected validator like so:

from jsonschema.validators import validator_for

Validator = validator_for(schema)
Validator(schema, format_checker=Validator.FORMAT_CHECKER).validate(instance)

[codecov]

Codecov Report

Merging #905 (36291ae) into main (6787b21) will decrease coverage by 0.02%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             main     #905      +/-   ##
==========================================
- Coverage   97.05%   97.02%   -0.03%     
==========================================
  Files          20       20              
  Lines        3290     3295       +5     
  Branches      457      458       +1     
==========================================
+ Hits         3193     3197       +4     
  Misses         77       77              
- Partials       20       21       +1     

Impacted Files	Coverage Δ
jsonschema/validators.py	97.09% <80.00%> (-0.33%)	⬇️
jsonschema/protocols.py	87.50% <100.00%> (+0.54%)	⬆️

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[TiborVoelcker]

Hey @Julian. Just so you know: altough I have been coding with Python for a couple of years, this is my first PR to a public repo, so I do not know what I am doing^^

I only did a few minor changes, but it did seem to work with the syntax you provided in #861.
Please let me know if I missed something, or whatever else.

[Julian]

Wonderful, I'm sure it's great! Will have a look, do ping me if I don't do so and give you feedback in the next few days or so as I've got a lot of different things all stealing attention but yeah really appreciated!

[Julian]

@TiborVoelcker Sorry for the delay in merging. This was perfect, well done.