Support passing through all modules

[cretz]

What was changed

• Added passthrough_all_modules dataclass field to temporalio.worker.workflow_sandbox.SandboxRestrictions class, defaulted to false that, when set to true, will be as if all modules are passed through
• Added with_passthrough_all_modules helper method to that same class to return a new instance with that value set to true
• Some README updates clarifying this new setting

Checklist

1. Closes [Feature Request] Make option for "all passthrough" (i.e. disable import isolation part of sandbox) #691

[cretz]

@dandavison - this one good? Will merge in a bit if so

[cretz]

Assuming with the minor comment and the other approval that we're good, merging...