fix: refresh token flow uses x-www-form encoding

[JorTurFer]

Description

We have seen that during refresh_token grant type, the parameters are sent in query string and are logged by our monitoring systems. For token requests (specially over internet) x-www-form encoding should be used and currently it's used for code grant type too.

This PR updates the refresh_token grant type to use x-www-form

Checklist

• [x] Code format was applied: make fmt
• [x] Examples were added / adjusted (see e.g. here)
• [x] Docs are up-to-date: make generate-docs (will be checked by CI)
• [x] Unit tests got implemented or updated
• [x] Unit tests are passing: make test (will be checked by CI)
• [x] No linter issues: make lint (will be checked by CI)