OpenTracing / Context propagation support

As suggested by @jeffschoner-stripe and @drewhoskins-stripe, I am creating a github issue to discuss the topic of context propagation for the Ruby SDK.

# Tracing Proposal

## Why
Distributed applications are more difficult to measure and debug due
to the distributed nature of their processing. One cannot simply
attach to the running process with a debugger and capture state
since that state may be spread across multiple processes or systems.
Temporal, by its nature, is distributed. Any application written using
Temporal will also be distributed. In order to trace through the
behavior of the business logic implemented as workflow(s) and
activit(ies) we should provide support for OpenTracing and other
related tracing mechanisms.

The [Java](https://github.com/temporalio/sdk-java/tree/master/temporal-opentracing), [Go](https://docs.temporal.io/docs/go/tracing/), and [Typescript](https://docs.temporal.io/docs/typescript/interceptors) SDKs all support OpenTracing today via
context propagation. The Ruby SDK can support Tracing today without
any changes merely by allowing the programmer to pass the Tracing
context as an argument to every workflow and activity. This is sloppy
and error prone, so we should mimic the facilities exposed by the
aforementioned SDKs and allow the context to propagate automatically
using the Temporal headers.

## What
This defines the interface for passing in OpenTracing contexts to
the Temporal system. Upon configuration, the propagation of this
information should be automatic and transparent to the programmer.
Aside from initial configuration, methods should be exposed on
the workflow and activity that allow for adding to the existing
(if any) context.

### Workflow Configuration
Will fill this out in next edit... thinking about it.

No files or tests are named; begin by reviewing the Ruby SDK's workflow and activity configuration and how Temporal headers are handled. The issue is not ready to implement yet: the workflow configuration interface is still unfinished, and completion would require an agreed design for automatic context propagation plus workflow and activity methods for extending existing context.

Written by the indexing model from the issue text.