* Allow a whitelist for the context parameter check
This allows users to configure a set of types that may appear before
`context.Context`.
Notably, I think this rule is useful for allowing the `*testing.T` type
to come before `context.Context`, though there may be other uses (such
as putting a tracer before it, etc).
See #605 for a little more context on this.
Fixes#605
* Save a level of indentation in context-as-arg validation
We can unindent if we make the above check more specific
* refactoring taking into account chavacava's review
Co-authored-by: chavacava <salvadorcavadini+github@gmail.com>
