Design observable `traces` configuration syntax

[jfallows]

As with other first class concepts in zilla.yaml, such as vaults and guards, we need a general concept of traces, independent of how they are stored locally or transmitted remotely, with more specific implementation for observability standards such as OpenTelemetry.

This can be used to follow the trace of Zilla streams through the bindings in zilla.yaml, and also provide context for remote invocations so that a distributed trace can be observed across the different systems where data flows.