Improve header serialization and propagation

[jeffschoner-stripe]

Summary

This contains two improvements to headers, both of which change the behavior of workflow headers:

First, headers are now by default propagated to the next run of a workflow on continue-as-new. This can be overridden by passing options: {headers: overridden_headers} to workflow.continue_as_new if desired.

Second, to set headers, you no longer need to use the generated proto classes. Instead, you can simply pass an ordinary hash, which will be automatically serialized into the proper proto. This makes the use of headers more ergonomic and eliminates the need to use the lower-level protobuf abstraction in code that uses temporal-ruby. However, this is a breaking change. If you are currently passing protobuf objects in for headers, you will need to instead pass a hash.

Testing

The continue-as-new integration test will now check that headers are propagated to the next run, as it already did with memos. The metadata workflow test now passes headers in the new way.

pushd examples
rspec spec/integration/continue_as_new_spec.rb
rspec spec/integration/metadata_workflow_spec.rb
popd

Existing unit tests have been updated to work with passing an ordinary hash for headers:

rspec spec/unit/lib/temporal/connection/serializer/continue_as_new_spec.rb

[antstorm]

Looks great, thank you, @jeffschoner-stripe!

One note — do you think it's feasible to support the legacy behavior while adding a deprecation message?

You can probably switch on a class name, however it might not be worth it as I don't think there are lots of people who are dependent on it. Let me ask in Slack

[jeffschoner-stripe]

I've added another commit for this back-compat. Feel free to merge it, or we can remove it if there's no need to support that.

[antstorm]

Perfect, thank you @jeffschoner-stripe !