Adding UpdateActivityOptions, first iteration

[ychebotarev]

What changed?

First iteration for UpdateActivityOptions. Contains most of the logic for UpdateActivityOptions, working with proto, updating options, usage of updateMask, unit tests, etc. Functional tests will come next. To reviewers -

• check the main logic in updateactivityoptions/api.go.
• check if anything fundamental is missing.

Known missing parts - reroute activity to the new TaskQueue is TaskQueue name was changes, activity is scheduled but not started yet. Knows issues with proto - I was not able to make protorefrect work with "messages as a pointers", IsValid (should return false if field is a pointer and nil) is just not working. As the result code is a bit less general. Though more readable.