chore: allow both string and []string in aud field

[m1pl]

According to the specification, the aud field can either be an array of strings or a single string. Oathkeeper only supports arrays, which breaks the integration with IAM tools like Keycloak and forces users to use ugly hacks like setting an additional fake audience. This change adds support for strings to Oathkeeper.

Related issue(s)

@aeneasr
#491
#601
#792
#810

Checklist

• I have read the contributing guidelines.
• I am following the
  contributing code 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.

[codecov]

Codecov Report

Merging #822 (d50c7a2) into master (1f1f03a) will increase coverage by 0.05%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #822      +/-   ##
==========================================
+ Coverage   62.42%   62.47%   +0.05%     
==========================================
  Files         102      102              
  Lines        4793     4813      +20     
==========================================
+ Hits         2992     3007      +15     
- Misses       1528     1531       +3     
- Partials      273      275       +2     

Impacted Files	Coverage Δ
...peline/authn/authenticator_oauth2_introspection.go	81.38% <75.00%> (-0.76%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1f1f03a...d50c7a2. Read the comment docs.

[zepatrik]

Thanks, the code itself looks good to me 👍

[aeneasr]

Looking great :) As @zepatrik pointed out please add a test :)

[m1pl]

@zepatrik @aeneasr I added tests.