How to warn users if we reject a fleet configuration update.

In #8220 we are discussing ignoring bad configuration updates (specifically around invalid/empty sampling policies) which can cause APM to either go into a boot loop on cloud or effectively run in `denyall` mode.

Opening this issue to ensure we find a way that we don't do so silently:

* The absolute minimum we should do is log this rejection.
* We need a mechanism to either reject a configuration update or inform Fleet that we've only partially accepted a configuration update. (cc @elastic/fleet).
* Explore emitting warnings from apm-server
1. We could emit warnings in a new datastream that the @elastic/apm-ui folks could use to actively warn users of ongoing warnings.
2. Create a dedicated warnings API in apm-server that @elastic/apm-ui could query and we can use in our diagnostics tooling.