Allow setting *additional* memo values in ContinueAsNewOptions

Is your feature request related to a problem? Please describe.
We have an abstraction for starting workflows that sets some standard memo values. I have a new use case where I'd like to continue-as-new a workflow execution (started using the above mentioned abstraction) but when doing so, set a few additional memo values that the next run would read. Today, ContinueAsNewOptions allows setting the entire memo map (using ContinueAsNewOptions.newBuilder().setMemo()). Thus, in order to add new memo values to the next run while preserving existing values, we need to extract all the values (which requires knowing the entire key set) and then merging those with the new values. There is currently no API to either list all keys present in the memo, or to fetch the entire memo structure.

Describe the solution you'd like
Ideally, ContinueAsNewOptions.Builder should provide an addMemo(key, value) method which overrides the value in the current execution's memo if present, or else, adds it to the next workflow execution.

Describe alternatives you've considered
Alternative approaches:
1/ add the memo values to the next workflow's arguments (leaky abstraction)
2/ maintain a list of all known memo keys and use that to extract the entire memo when building ContinueAsNewOptions.

Start with ContinueAsNewOptions.Builder and the existing setMemo() API, then inspect how the current execution's memo is accessed and how continue-as-new options are tested. Done means an addMemo(key, value) API can preserve existing memo values while overriding or adding the specified key for the next workflow run.