Invocation state proxy for Function Executors and new API for functions

[eabatalov]

We don't want to have Indexify Server API credentials in Function Executors because it's a security risk if Function Executors run not trusted code. To achieve that Executor provides a gRPC API to Function Executor to get/set graph invocation state. Once a request is recieved Executor finds invocation ID of the requesting Function Executor and does Indexify Server API call to get/set and invocation state key.

There are a few customer function facing API changes here:

Instead of calling get_ctx().set_state_key() and get_ctx().get_state_key() they call
get_ctx().invocation_state.get() and
get_ctx().invocation_state.set().

This intoduces a separate API for graph invocation state which is more clear than adding unbounded number of methods into GraphInvocationContext object returned by get_ctx(). We're thinking about long term extensibility of it here. Also as local graph uses LocalInvocationState implementation and remote graph is using ProxiedInvocationState we'll just need to forward get_ctx().set_state_key() and get_ctx().get_state_key() calls into these implementations anyway.

Another change is that set(key, value) now accepts any value that is serializable using CloudPickle. This is provides a consisten "no surprises" UX because we're using CloudPickle for function inputs and outputs. Supporting this required small changes on Indexify Server side as previous JSON was explicitly used at HTTP protocol and storage layers. Now it supports arbitrary binary and textual formats for invocation state values.

Finally get(key) now returns Optional[Any] so None is returned if the key wasn't set yet. This allows the customer code to decide what to do in this case.

Testing:

make fmt
make test

Contribution Checklist

• If the python-sdk was changed, please run make fmt in python-sdk/.
• If the server was changed, please run make fmt in server/.
• Make sure all PR Checks are passing.