feat: use uri-reference for errors redirect to allow relative urls

[xlanor]

This is related to ory/kratos#617

Similar to that of the original PR, we are running a multi-domain setup for our cluster(s).

The original PR for kratos greatly assisted with our deployment process.

However, we are also utilising oathkeeper as a proxy in our cluster, and as .oathkeeper.yml's errors:handlers:redirect:config:to only allows absolute URIs, we have had to deploy an instance of oathkeeper per domain.

This PR aims to allow for a relative error redirect URL in the oathkeeper configuration file.

I have not built a container to test this yet as I am currently still on leave, but once I'm back in the office I will build it to test directly - it's my first time submitting a PR here, so I would like to get some eyes on it :)

Related issue

Proposed changes

Checklist

• 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
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• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments

[CLAassistant]

All committers have signed the CLA.

[zepatrik]

I'm not quite sure if I read the diff correctly but can you make sure there are all old test cases as well as the new ones? I'd expect 4 cases in total, for each of absolute and relative a 301 and 302 case.

[zepatrik]

Thanks, LGTM.

[aeneasr]

Awesome, thank you for your contribution!