Export class declarations from the types directory

[hossam-nasr]

Prelude to #458, part of #416. This PR adds class declarations exported from the root of the package (living in the types directory), for relevant public-facing classes. Specifically, it exports declarations for the following classes:

• PurgeHistoryResult
• EntityStateResponse
• HttpManagementPayload
• EntityId
• DurableOrchestrationStatus
• RetryOptions
• ManagedIdentityTokenSource
• AggregatedError
• DurableError
• DummyOrchestrationContext
• DummyEntityContext

Furthermore, it also moves the OrchestrationRuntimeStatus enum to the orchestrationTypes file.

[davidmrdavid]

PR mostly makes sense. Just a question at this time.

[ejizba]

Just the nit about the message? thing left, but otherwise LGTM