Add support for calling input plane functions

[thecodingwizard]

This PR adds libmodal support for calling input plane functions. This requires a few changes:

1. Support multiple ModalClientStubs pointing to different server URLs.
2. Propagate x-modal-auth-token from FunctionGet to the input plane RPCs.
3. Implement the new set of RPCs for the input plane.

We will need to add support for retrying internal failures next.

Note that this PR cannot be merged until we enable input plane functions in production; otherwise, the tests will fail.

[thecodingwizard]

Note that the way we propagate auth tokens is a little different in Go vs. Typescript.

In Typescript (and Python), we propagate auth tokens via auth middleware / a grpc interceptor in client.ts.

In Go, we propagate auth tokens via context in function.go.

I'm open to changing Go's implementation to also propagate auth tokens via grpc interceptors in client.go for consistency if we think that's better, but it might be a bit trickier since Go is multithreaded? Regardless, we can always change our minds later.

[rculbertson]

@ekzhang I rebased this PR on main, and updated it to be compatible with the changes made since it was first opened. So far I've made the changes only to the typescript part. If those changes seem good, I'll go ahead and take the same approach in the Go code.

In the code in main, #execFunctionCall returns a function call id to .remote. That then calls pollFunctionOutput with the function call id, which calls FunctionGetOutputs. For the input plane, we don't have a function call id - we have an attempt token. We need .remote to call AttemptAwait and pass it the attempt token.

To handle these two cases, I created a class called FunctionOutputPoller. This replaces the pollFunctionOutput function. Within #execFunctionCall, we create an instance which is configured to call either the input plane or the control plane. In .remote we call FunctionOutputPoller.poll() which will call the right one.

Hope that makes sense! Happy to chat IRL if that's easier!

[rculbertson]

After starting to implement client retries, I realized some more refactoring was needed. I opened this PR #30, which is based on top of this one. In that one, I removed the FunctionOutputPoller class and replaced it with the InputStrategy interface (and the classes which implement it). See that PR's description for more info.

[rculbertson]

Closing in favor of:

• Add InvocationStrategy for sending inputs to control plane #31
• Add support for input plane invocation #32