Add ability to inject global inputs `__foo` variables

[elijahbenizzy]

Is your feature request related to a problem? Please describe.
There are some inputs that everyone might want. Tracers, etc... We should be able to inject it.

Describe the solution you'd like

Thinking something like this:

(
    ApplicationBuilder()....with_global_inputs(
    __langfuse_tracer=(app_id, ...: LangFuse(...), NoOpLangfuse())) # other parameters/whatever you call for langfurse
    .build()
)

Or some other way to provide a no-op integrator.

Describe alternatives you've considered
Could do it with inputs manually.

[andreahlert]

Labels migrated to new taxonomy (#692).

[andreahlert]

Closing as stale. No discussion or external demand since it was opened. Can be reopened if this becomes a real need.