Export GetRegisteredWorkflowTypes so I can use in shadowtest.

[ZackLK]

Exported the GetRegisteredWorkflowTypes() function for public use.

In order to know what function types have been registered and be able to iterate them, and run the WorkflowShadower on each exported function type.

[coveralls]

Pull Request Test Coverage Report for Build 0184f00f-f5e4-4bf6-aca0-20c84cbd68db

• 5 of 12 (41.67%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 64.262%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
workflow/workflow.go	0	3	0.0%
internal/workflow.go	0	4	0.0%
<!--	Total:	5	12	41.67%	-->

Totals
Change from base Build 0184e8f9-af9f-4766-b6a9-3fb5e0401f70:	-0.02%
Covered Lines:	12684
Relevant Lines:	19738

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💛 - Coveralls

[Groxx]

👍 nice and easy after the cleanup. thanks! I think there's a good chance we'll want an activity version eventually, and probably some per-worker/registry versions as well... but I'm not sure how far we can take this without making some big breaking API changes. The rest is probably better to wait for v2, though in general this seems like a useful thing to expose.