Allow to override path for schema matching

[ahx]

If your API is deployed at a different path than what's defined in your OpenAPI schema, you need to overwrite the path that is used for request/response validation. #345 added an env variable to do that. But being able to do that without adding a custom middleware or sub-classing the existing middewares would be helpful.

Follow-up of #347 (comment)

[ahx]

A config option for this was just released in 2.7.0. It works like this:

oad = OpenapiFirst.load('openapi.yaml') do |config|
  config.path = ->(req) { request.path.delete_prefix('/api') }
end
use OpenapiFirst::Middlewares::RequestValidation, oad

Thanks for the work on this @kaorukobo!