Add basic middleware and endpoints for debugging services

[orlade-anz]

Adds recording of upstream and downstream HTTP requests to a memory store, and endpoints to explore the metadata and overlay it on a sequence diagram.

• /-/trace provides a list of trace IDs that have been recorded (expected header is traceId)
• /-/trace/{traceId} provides details of the up- and downstream requests
  • Hovering over the labels on the sequence diagram displays the metadata for that request/response

Limitations:

• Sequence diagram is currently hardcoded to the PaymentServer demo app and lacks return types
• Modifying the SVG is inefficient, and takes a couple of seconds

Depends on https://github.com/anz-bank/sysl-go/pull/195

[anzdaddy]

This PR is quite stale. We'll need to get it merged from master or rebased against it.