Add CI credentials for FullStory integration tests

[joshdover]

In #102390 we added an integration test that is not currently enabled in CI. To get this enabled we need to:

• Get the appropriate credentials populated into the FULLSTORY_ORG_ID and FULLSTORY_API_KEY env vars on Jenkins
• Add x-pack/test/cloud_integration/config.ts to one of the functional test scripts that are executed by Jenkins in test/scripts/*.sh
• Verify that the test gets run by inspecting the JUnit output in a CI run

[TinaHeiligers]

The ops team responded that we need to export the needed env vars from src/dev/ci_setup/setup_env.sh. The file already reads the injected vars from Vault.

Other env vars are read from there in https://github.com/elastic/kibana/blob/master/src/dev/ci_setup/load_env_keys.sh. I think that file also needs updating.

I've created an issue to add the vars to Vault with infra but they said that the @elastic/kibana-operations should be able to help out with adding these to the Vault. @spalger thanks for offering to add those once the final values are confirmed with the Cloud folks!

cc @joshdover