Add getMemo to Workflow

[vkoby]

What was changed

Added a getter for Memo in Workflow

Checklist

1. Closes Missing getMemo functionality in WorkflowInfo #512

2. How was this tested:
   Unit Test

[Spikhalskiy]

Memo is exposed to the user as setMemo(Map<String, Object> memo) (

sdk-java/temporal-sdk/src/main/java/io/temporal/client/WorkflowOptions.java

Line 207 in b642a75

public Builder setMemo(Map<String, Object> memo) {

)

So, getMemo should return the same deserialized structure, not a raw Protobuf entity.

[Spikhalskiy]

with minor comments, but please address them before merging.