Track external payloads stats for workflow execution

[simvlad]

What changed?

Keep the total number and the size of the external payloads per the workflow execution

Why?

We are working on building the support for external payloads in SDK, which are stored outside of Temporal. We'd like to be able to show the total size and the number of external payloads in the given workflow execution.

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

Potential risks

N/A

[yycptt]

I think state-based replication logic in workflow_state_replicator.go also changes HistorySize and needs to be updated as well. Haven't finished the review yet, will continue tomorrow.

cc @xwduan @yux0 Please help review this PR as most of the changes are actually on the replication side.