Improve importability of Beats in other code bases

[leehinman]

Beats code base currently has a number of assumptions that make it very difficult to import beats into another code base and makes it very difficult to run multiple instances of beats within the same process.

• libbeat/common/reload uses a package global variable for the Registry. This means you can't have 2 different beats with 2 different registries in the same process, this needs to be fixed. refactor libbeat/common/reload to not use global variable #40622
• filebeat modules and metricbeat modules register metrics in the same namespace. Each beat needs it's own namespace to prevent collisions. refactor filebeat and metric module namespace #40660
• libbeat sets command line flags here and here, if libbeat is imported in another code base you can have conflicts with flags. This needs to be changes so libbeat can be imported without setting any flags.
• Refactor Beat instantiation and setup #40659
• libbeat sets fleetmode by looking at CLI flags, this needs to be fixed. refactor fleet mode detection and storage #40667

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[blakerouse]

Final importability issue that was linked to this issue was fixed here - #40993