Communicate shouldContinueAsNew to Workers

[lorensr]

Is your feature request related to a problem? Please describe.

When writing a long-running Workflow, I'd like to know when to Continue As New.

Describe the solution you'd like

I'd like to have SDKs provide both of these to Workflows:

• eventHistoryLength (SDKs should expose history length and size via Workflow info features#16)
• shouldContinueAsNew

The former is the last event ID. The latter can either be:

1. Set by the SDK: for instance shouldContinueAsNew = eventHistoryLength > 10_000
2. Communicated by the Server (in an event header?). This might be preferable, as the Server has more knowledge—for example, it might be able to keep a running total size of a Workflow's Event History, and set shouldContinueAsNew=true before 10k events if the Payloads are large. And maybe @mfateev had other reasons for suggesting this route?

We should give Workflow.isContinueAsNewNeeded flag to the SDKs. And it should be coming from the service.

https://temporaltechnologies.slack.com/archives/C01FT8U10GK/p1643400086752909?thread_ts=1643398281.925129&cid=C01FT8U10GK