Fix events iteration

[klizhentas]

Description

Iteration

This change will not require any large refactor, but will require API update.

Implement a new GRPC Events api that adds StartKey in addition to limit. This StartKey behavior should be similar to DynamoDB query API that provides pagination:

// GetEventsRequest replaces two old methods: GetEvents and SearchEvents
message GetEventsRequest {
   // Namespace, if not set, defaults to 'default'
   Namespace string
   StartDate Timestamp
   EndDate Timestamp
   // EventType is optional, if not set, returns all events.
   // Current DynamoDB implementation is incorrect, it filters out events on the client
   // Instead it should activate Scan method to return the full set.
   // Check the behavior for firebase
   EventType string
   Limit int64
   StartKey string // When supplied the search will resume from the last key
}

// GetSessionEvents replaces GetSessionEvents
message GetSessionEventsRequest {
   // SessionID is a required valid session ID
   SessionID string
   // EventType if set will look for an event type
   EventType string
   Limit int64
   StartKey string // When supplied the search will resume from the last key
}

returned value:

message Events { 
    Items repeated oneof Event
    LastKey string // key returned if the result set is incomplete
}

API and Client library

Add as official API to teleport/api client library code

[alex-kovoy]

related #5388

[alex-kovoy]

@klizhentas lets not forget a filter|query field

[klizhentas]

@alex-kovoy what filter? this is not the same as search events API I think, this is a separate one for enumeration. @russjones we might port the Search endpoint to GRPC as well, I will write it up

[klizhentas]

@russjones @xacrimon ping me about this when you'll start hacking on this. #6160