Rename invoke to invokeMethod for ActorProxy

[artursouza]

We have recently renamed invokeActorMethod() to invoke() in ActorProxy, after discussions within maintainers, we decided to rename it to invokeMethod() instead.

Parent: dapr/dapr#2535

Related: #409

[artursouza]

/cc @arghya88 We discussed the renaming again and we decided to keep invokeMethod() for service invocation but rename invoke() to invokeMethod() for ActorProxy to keep consistency in naming.

[He-Pin]

/assign