Add worker and client plugin system

# Summary

Add a plugin system for TemporalWorker and TemporalClient that allows intercepting configuration and lifecycle, matching the C# and Ruby SDKs. Documentation for plugins can be found here https://docs.temporal.io/develop/plugins-guide#built-in-workflows

Start with the TemporalWorker and TemporalClient entry points, then read the Temporal plugins guide and compare the corresponding C# and Ruby SDK implementations. Define what configuration and lifecycle interception must support for both Swift types, with completion shown by a working plugin system covering worker and client behavior.

Written by the indexing model from the issue text.