feat: flush refresh tokens for service oauth2/flush

[naveenpaul1]

Related issue

This changes are done based on the discussion happend on thread https://github.com/ory/hydra/issues/1574#issuecomment-736684327

@aeneasr @Demonsthere

Proposed changes

flush expired refresh token along with access_token in /oauth2/flush service call, where condition remains same as access toke table deletion query

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
  security@ory.sh) 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.

Further comments

[CLAassistant]

All committers have signed the CLA.

[aeneasr]

This looks really good, thank you! @Benehiko can you take a look as well please?

What would be perfect as well would be a small test to check if the behaviour works as expected - you can probably add some code to this test: https://github.com/ory/hydra/blob/master/oauth2/handler_test.go#L131

[Benehiko]

@aeneasr LGTM :)

[naveenpaul1]

updated the PR with unit test for access token scenario.
@aeneasr @Benehiko : please review

[aeneasr]

Awesome, thank you! 🎉 Your contribution makes Ory better :)