Decouple inbox/exclusive lock usage from invocation target

[slinkydeveloper]

Right now we couple the inbox usage in the partition processor to the invocation target, see

restate/crates/worker/src/partition/state_machine/mod.rs

Line 630 in 674501a

if metadata.invocation_target.invocation_target_ty()

I propose to add a new field to ServiceInvocation, e.g. locking_semantics, that is optional and when present, enables the usage of the exclusive service lock and of the inbox. The first variants would be:

enum LockingSemantics {
  Lock { key: String }
  NoLock
}

Names to be defined/open to suggestions.

This new field would then be used by the virtual object exclusive handlers, replacing the current coupling to invocation target, and by #1299 as well.
It might turn out to be useful for #12 too, if we add another variant ReplaceExistingOne or similar.