Correlation Middleware should allow correlation ID to be passed in via a custom incoming middleware

GitLab.com is now running behind a CloudFlare WAF.

This infrastructure includes a cf-request-id header.

LabKit should allow the application to pass in a WithIncomingHeader('...') option, which would then be used to set the correlation_id for further propagation through the application (from the boundary onwards the existing headers would be used).

This could be used to configure Workhorse, running on GitLab.com, to "adopt" and propagate the Cloudflare cf-request-id header as our correlation id.

cc @stanhu @msmiley