Email/Query filter by keywords

As an user i want to be able to select messages by their keywords using the 'hasKeyword' and 'notKeyword' filters.
The following system flags are handled
 {{"$Draft"
"$Seen"
"$Flagged"
"$Answered"
"$Forwarded"}}
user keywords should be handled too

Why

As an user i want to be able to select the mail in a mailbox like this :
 {{[[ "Email/query",{
"accountId": "ue150411c",
"filter":

{ "hasKeyword": "$Flagged" }

}, "0" ]]}}

HOW

• accept the filters 'hasKeyword' and 'notKeyword' in the filter condition from the request
• filter the query with the provided keywords

DOD

• write integration demonstrating that a user can filter its emails by supported system keyword
• write integration demonstrating that a user can filter its emails with valid user keyword