Adding WorkflowReplayer and bumping version to 2.2.0

- Adding a WorkflowReplayer for Ruby Flow
- Bumping version to 2.2.0

WorkflowReplayer is an AWS Flow Framework utility that is used to 'replay' a workflow history in the decider against the workflow implementation. It is a useful debugging tool.

Usage -

```ruby
# Create an instance of the replayer with the required options -
require 'aws/replayer'
replayer = AWS::Flow::Replayer::WorkflowReplayer.new(
  domain: '<domain_name>',
  execution: {
    workflow_id: "<workflow_id",
    run_id: "<run_id>"
  },
  workflow_class: WorkflowClass
)

# Call the replay method (optionally) with the replay_upto event_id number -
decision = replayer.replay(20)
puts decision.inspect
```