Optionally allow overriding the policy class for permitted params

[Numie]

Fixes #742

To do

• I have read the contributing guidelines.
• I have added relevant tests.
• I have adjusted relevant documentation.
• I have made sure the individual commits are meaningful.
• I have added relevant lines to the CHANGELOG.

PS: Thank you for contributing to Pundit ❤️

[Burgestrand]

Hi, thanks for the pull request!

I adjusted the PR description to use the PR template we've got going on. This change would need a CHANGELOG entry.

On another note, I feel the scope of this PR is somewhat inflated by the moving of the permitted_attributes implementation into Pundit::Context, but I'm not sure why that was done. I think that can be skipped entirely, to keep the change small.

I also notice that pundit_params_for(record) also looks up a policy, and I wonder if that policy shouldn't be the same policy if you pass in policy_class. That would necessitate a change there too.

I'll ponder the whole Pundit::Context and permitted parameters relationship in the meanwhile, too.