feat(gate-web): Add delete session tokens from redis endpoint

[juangod-wise]

When Spring Boot is upgraded and old tokens no longer work, users get 500's until they delete the cookies from their browser, which causes bad user experience.

A way around this is to delete the spring:session* keys from Redis, as explained here:

https://spinnaker.io/changelogs/1.34.0-changelog/#spring-boot-2615

$ redis-cli keys "spring:session*" | xargs redis-cli del

In our case, we don't have CLI access to our Redis instance in Production. Because of this, we wrote a Gate endpoint that deletes the keys from Redis, so it can be triggered in Production with a curl call:

curl -H "Cookie: SESSION=...." -i -X POST https://spinnaker-host/api/v1/auth/deleteSessionCache

I'm sure many other users of Spinnaker have the same issue and could benefit from this.

[spinnakerbot]

We prefer that non-test backend code be written in Java or Kotlin, rather than Groovy. The following files have been added and written in Groovy:

• gate-web/src/main/groovy/com/netflix/spinnaker/gate/services/SessionService.groovy

See our server-side conventions here.

[spinnakerbot]

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• 639c99d: Add delete session tokens from redis endpoint

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