Support `workspace/executeClientCommand`

[BoykoAlex]

JDT LS has LSP extension workspace/executeClientCommand. The command is sent as a request message to the client. The client executes the command.

This command is currently being used to communicate classpath entries from JDT LS to other LS requiring classpath data. The communication is done via workspace/executeCommand for client -> server communication and `workspace/executeClientCommand for server -> client communication. As the result JDT LS to LS "X" is done through the client using command registration and execution.