feat: async backchannel logout

[aarmam]

This pull request introduces feature to execute backchannel logout asynchronously.

Use case: User should not have to wait for backchannel requests to finish (4x30sec, with potential connection timeout 1min on each request), when performing logout from all applications using GET /oauth2/sessions/logout or from specific application by using DELETE /oauth2/auth/sessions/consent?trigger_backchannel_logout=true (#2844)

Will add configuration option if needed.

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• 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
  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

Tests and documentation will be commited after inital acceptance of the proposed feature.

[codecov]

Codecov Report

Merging #2849 (cc60449) into master (1590542) will decrease coverage by 0.23%.
The diff coverage is n/a.

❗ Current head cc60449 differs from pull request most recent head c1161aa. Consider uploading reports for the commit c1161aa to get more accurate results

@@            Coverage Diff             @@
##           master    #2849      +/-   ##
==========================================
- Coverage   79.64%   79.41%   -0.24%     
==========================================
  Files         112      112              
  Lines        7955     7885      -70     
==========================================
- Hits         6336     6262      -74     
- Misses       1217     1222       +5     
+ Partials      402      401       -1     

Impacted Files	Coverage Δ
consent/strategy_default.go	69.48% <ø> (-1.04%)	⬇️
oauth2/handler.go	68.27% <ø> (ø)
cmd/cli/handler_janitor.go	78.88% <0.00%> (-2.73%)	⬇️
oauth2/trust/handler.go	75.86% <0.00%> (-0.41%)	⬇️
persistence/sql/persister_grant_jwk.go	81.08% <0.00%> (-0.34%)	⬇️
hsm/manager_nohsm.go	0.00% <0.00%> (ø)
oauth2/trust/manager.go	100.00% <0.00%> (ø)
oauth2/trust/validator.go	100.00% <0.00%> (ø)
oauth2/fosite_store_helpers.go	100.00% <0.00%> (ø)
... and 2 more

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[aeneasr]

It looks like this PR is somewhat broken? There is only one file that has changes unrelated to the PR's title

nvm :)

[aeneasr]

I think this makes sense! Could you please add some documentation that explains that this runs async?