Send APMConfig to registered apm reloaders when the config has changed

[ycombinator]

Describe the enhancement:

Taken from elastic/apm-server#11381:

The client.Expected struct attached to each unit contains an APMConfig field:

type Expected struct {
	Config    *proto.UnitExpectedConfig
	Features  *proto.Features
	LogLevel  UnitLogLevel
	State     UnitState
	APMConfig *proto.APMConfig
}

When this field's value changes, the change is signaled using TriggeredAPMChange

	// TriggeredAPMChange indicates that the APM configuration has changed
	// This constant represents a single bit in a bitmask
	TriggeredAPMChange // apm_config_change_triggered

We should detect this change and send the changed APMConfig to all reloaders registered under a new libbeat/common/reload registry key, "apm".

See also elastic/apm-server#11381 (comment), elastic/apm-server#11381 (comment), and other comments in that issue for more context / background.

Describe a specific use case for the enhancement or feature:

elastic/apm-server#11381

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)