chore: Deprecate defaultSinks

[mjameswh]

What changed

• Deprecate defaultSink() and LoggerSinks. SDK's logger sink is now forcibly injected. Registering a custom logger sink named defaultWorkerLogger is still supported, but discouraged and deprecated.
• Refactored sink code internally.

Why

• These changes adds up to better DX, notably by:
  • Promoting clear usage patterns regarding Workflow logging that works out-of-the-box;
  • Discourage the use of insufficiently documented features (eg. possibility of redirecting workflow logs by registering a custom defaultWorkerLogger sink);
  • Remove the requirement that users should call defaultSinks() if they were to add custom sinks, which was error prone and a recurring source of support question.
• Prepare the code base for other, similar upcoming refactors and some features (eg. custom metrics, context forwarding, etc).

[Sushisource]

LGTM. Comments aren't blocking.

[bergundy]

Overall LGTM, thanks for clearing up this mess.