feat: Add url_param config option to redirect error handler.

[marcinkoziej]

This change introduces a url_param config option for redirect error handler.
If it contains a url paramter name, the redirect url will have this parameter
set, containing the current url (from which Oathkeeper has redirected the user).

This can be useful in passing the return_to url to Kratos, so user can be
redirected to the page they initially wanted to access after a successfull sign in.

Related issue

https://community.ory.sh/t/how-to-use-the-return-to-query-string-with-oathkeeper-kratos/2149/6

Proposed changes

Add url_param option to redirect error config, which configures a name of paramter that should contain current url (the url that failed the pipeline).

If url_param is given, the redirect error handler will parse the to: url and append url_param parameter with current url.

Checklist

⚠️ Disclaimer: I am not a Golang coder. I'd appreciate the feedback on this patch and I intend to make it better. I have not written documentation yet – unsure if my approach is ok.

• I have read the contributing guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

[CLAassistant]

All committers have signed the CLA.

[zepatrik]

Nice, thanks for your contribution 👍
I just found some go specific mistakes.

[marcinkoziej]

Thank You so much for the fixes! Oyoyoy that shadowing of a library was nasty >:-}

[marcinkoziej]

Sorry, I clicked accept suggestions which resulted in multiple commints. This PR should be merged-squashed probably.

[zepatrik]

Nice thanks

[aeneasr]

Thank you, this looks great! I have some ideas how to improve this. Could you also please add some docs to: https://github.com/ory/oathkeeper/blob/master/docs/docs/pipeline/error.md#redirect ? :)

[aeneasr]

Let's rename this to return_to_query_param to make it more obvious :)

[marcinkoziej]

@aeneasr done

[marcinkoziej]

@aeneasr Hi, is there some way I can improve on this PR more?

[aeneasr]

Sorry, I'm currently on vacation which is why these reviews take a bit longer :) This looks great! Could you please resolve the merge conflicts? Then I'll be merging it right away! :)

[maxbublik]

@marcinkoziej Great job!
Looking forward to that feature very much.
Unfortunately I'm not a Go developer neither :(

[aeneasr]

Awesome 🎉

Thank you for your contribution!

[maxbublik]

@marcinkoziej @aeneasr Thanks a lot! I have compiled oathkeeper from source and the feature works!