[Feature Request] Better prevention of accidental `Runtime` use over forks

[cretz]

Describe the solution you'd like

Runtimes/clients/workers/etc do not work across forks. We need to do two things here:

• Prevent creating things with a runtime created in a different process
  • Basically do the exact same thing we did in Ruby: Prevent using SDK runtimes/clients/workers across forks sdk-ruby#301
• Allow users to opt-out of default runtime use
  • This means add something like Runtime.prevent_default() that will prevent any invocation of Runtime.default()
  • Clients and replayers and test environments use Runtime.default today which lazily creates runtime, but advanced users want to prevent accidentally doing this
  • Runtime.prevent_default() should error if it's called after a default has been created