Add support for rules in YAML format

[hypnoglow]

Proposed changes

This commit adds support for defining access rules in YAML format, in
addition to existing JSON format.

YAML is known as a superset of JSON.

This is useful in some cases, where YAML manipulation is preferable over JSON.

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
  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 necessary documentation within the code base (if appropriate)
• I have documented my changes in the
  developer guide (if appropriate)

[aeneasr]

This looks great! Only one nitpick :)

Oh and could you please document this in the oathkeeper docs, preferably in this file.

[aeneasr]

Oh and please run make format :)

[hypnoglow]

Oh and could you please document this in the oathkeeper docs, preferably in this file.

Sure, I will open a PR for docs soon.

[hypnoglow]

@aeneasr Done

[aeneasr]

Thank you!