Schedules: Make it possible to list buffered executions

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

A user creates a schedules and sets policy.overlap to ScheduleOverlapPolicy.BUFFER_ALL. At some point in time, user now want to "see" the list of buffered executions (ie. executions that have been queued to be executed, one at a time, after the current workflow execution completes).

As it is now, buffered executions appears neither in info.futureActionTimes, info.recentActions nor info.runningWorkflows, and no other field hints that some executions are currently buffered.

Describe the solution you'd like

• Add a field buffered_actions to the ScheduleInfo (code) structure returned by the DescribeSchedule API.

  In its simplest form, that field could be defined as:

   repeated google.protobuf.Timestamp buffered_actions = 9 [(gogoproto.stdtime) = true];
  

  Arguably, it could be marginally more useful to make that field a collection of some new protobuf object, containing both the original schedule time of the buffered execution and a flag indicating how that execution got queued up (eg. the execution got queued up because the schedule time was reached, because of a backfill request, or because of a "trigger immediately" request).

• Similarly add a field buffered_actions to the ScheduleListInfo structure, returned by the ListSchedules API.

Start with temporal/api/schedule/v1/message.proto, especially the ScheduleInfo structure referenced in the issue, and inspect the corresponding ScheduleListInfo definition and DescribeSchedule/ListSchedules responses. Determine how buffered executions should be represented, then verify that both APIs expose them in their returned schedule information.