[Access] Unify Event streaming endpoint on the execution data gRPC API

[UlyanaAndrukhiv]

Problem Definition

Currently, event streaming endpoint includes both startBlockID and startHeight. However, only one may be provided in a single request. This adds more complexity and code to the client.

Proposed Solution

Unify event streaming by separating SubscribeEventsendpoint into next endpoints:

• SubscribeEventsFromStartBlockID
• SubscribeEventsFromStartHeight
• SubscribeEventsFromLatest

As an example, SubcribeBlocks endpoints can be used.

Definition of Done

• Add separate messages for SubscribeEvents.
• Create new versions of the backend methods.
• Modify unit and functional tests covering the new endpoints.

[peterargue]

Please also add the messageIndex to responses. This was missed in the original design, and is needed to allow pagination as well as tracking responses for sparse data sets.