Add support for retrying internal failures

[rculbertson]

This is based on #15.

Note: Please review typescript only! Everything described below hasn't been implemented in go yet. If the typescript looks good, I'll do the same in go.

This builds on the above PR in the following ways:

• Retries up to 8 times on INTERNAL_FAILURE which can happen if inputs are lost for some reason.
• Adds InputStrategy interface, which provides a uniform API for sending inputs to both the input plane and control plane. The interface is implemented by two classes: InputPlaneStrategy and ControlPlaneStrategy. The client will decide which to use based on the function configuration - specifically if the function definition has an input plane region specified.

[ekzhang]

This seems like a pretty substantial first change — maybe we could write down what the plan is for input strategy and retries tracking long-term?

[rculbertson]

@ekzhang Yes, sorry for the large PR :(. If it makes things easier, we could start by reviewing #15 - that uses the simpler FunctionOutputPoller abstraction, which is replaced by InputStrategy in this PR. Happy to do whichever is easier.

I added this comment to the InputStrategy interface explaining that it can be deleted once we have moved all traffic over to the input plane.

[rculbertson]

Chatted with @ekzhang about this PR. To make it easier to review, I will open a new PR adding just the Strategy abstraction, implemented for just the control plane. It will not introduce any new functionality (i.e. no input plane support). We can then add input plane support in subsequent PRs.

[rculbertson]

Closing in favor of:

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