Added support for retry policies

[cgillum]

This PR covers the first two bullets of https://github.com/microsoft/durabletask-java/issues/18: task options and declarative retry policies.

Summary of changes:

• Introduced TaskOptions parameter for activities and sub-orchestrations
• Introduced RetryPolicy class which can be applied to TaskOptions to configure declarative retries.
• Started adding @Nullable attributes to help proactively identify NullPointerException is the code (I got hit by this a couple times during debugging).
• Added two integration tests: one for activity retry and one for sub-orchestration retry

Much of the design is inspired by the retry work I recently did for .NET Isolated: https://github.com/microsoft/durabletask-dotnet/pull/6.

Special thanks to @davidmrdavid for helping me figure out (without him knowing it) how to design retries in a language that doesn't support async/await. I borrowed the strategy for implementing retry policies from the Durable JS codebase.

After this PR gets merged, I plan to work on imperative retry policies, implemented in code.

[davidmrdavid]

My favorite help is that which occurs without me knowing :P . Glad to hear the JS approach was useful. Looking into the PR now