[Serverless] Global strip unknown keys doesn't work on nested objects

[afharo]

Related to #187006

We noticed that the global validation option stripUnknownKeys: true used in Serverless doesn't work as expected on settings defined in a nested schema.object:

elasticsearch.unknown.key: 1 # No issues (error log but Kibana continues spinning up)
unknown.key: 2 # Again... no issues (error log but Kibana continues spinning up)

xpack.dataUsage.autoops.api.tls.ca: abc # Failure! "[config validation of [xpack.dataUsage].autoops.api.tls.ca]: definition for this key is missing"

telemetry.labels.unknown: 123 # also fails!

#186547 might have been the culprit of the regression.

Let's add a test case to cover the scenario and find a way to fix it.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)