Add priority/weight (via header) to fiber-example

[sre-is-msg]

i.e. support the same paths and headers (i.e. x-user-plan) as this NodeSDK branch: https://github.com/StanzaSystems/stanza-js/pull/108/files

So that we can run the same loadtest script against either the node-sdk or go-sdk implemented apps and compare the results.

[sre-is-msg]

Picking this up next as I think having this will allow us to perf test Go SDK vs Node SDK.

[sre-is-msg]

Also add a suitable load-tester + script (a golang equivalent to artillery)

Contenders in descending order of likeliness to adopt:

• https://github.com/tsenart/vegeta
• https://github.com/grafana/k6
• https://github.com/rakyll/hey
• https://github.com/ddosify/ddosify

[sre-is-msg]

https://github.com/StanzaSystems/sdk-go/commit/04a814fa50cbb533a721ba34042ea7ace44f39de https://github.com/StanzaSystems/sdk-go/commit/559da2626928e638a4f645035f0274b27d34c693 https://github.com/StanzaSystems/sdk-go/commit/a07c0a75f491bea1afc5c8daf5efea886d6aee85

Done ✅