Move /src/apm.js to its own package

[pgayvallet]

In #92784, we migrated the /app/{id} endpoint from legacy to core. However the route handler needs to access the apm / rum configuration to inject it in the document's metadata. That causes an import from core to an undeclared source outside of its scope.

As we can't move the apm.js script to core (see #92784 (comment)), we should move it to a package to allow both the CLI and core to import it when they need to.

[elasticmachine]

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