feat: server events web hook

[gkc]

- What I did

• feat: Added experimental ability for server to send basic interaction event telemetry (client-to-server interactions, server-to-server interactions) to a configured web-hook, if any
• feat: Added telemetryEventWebHook to AtSecondaryConfig, enable it to be set from environment or config.yaml
• feat: Added ability for authenticated client to configure the web-hook uri. When this happens, the uri is persisted, and will override whatever was set via environment or config.yaml. Usage examples:
  • to set: config:set:telemetryEventWebHook=https://your.domain.testing.zone/hook
  • to reset to default (empty) : config:reset:telemetryEventWebHook
  • and to print current value : config:print:telemetryEventWebHook
• feat: removed testingMode guard in AtSecondaryConfig's subscribe and broadcastConfigChange methods since
  • (a) we now have configs that may be set on non-testing-mode servers and
  • (b) the guard, to prevent configs which require testingMode being set when testingMode is not enabled, has been added to the ConfigVerbHandler
• refactor: moved the telemetry initialization code into one method in AtSecondaryServerImpl
• build: updated pubspec to require later version of dart sdk because we are now using the enhanced enums feature for the ModifiableConfigs enum

- How to verify it All tests should pass