HHVM Debugger: Add support for an info command

HHVM Debugger: Add support for an info command

Summary:
This command is not part of the protocol and is experimental, but is needed for CLI feature pairity with hphpd.

Schema for new request and response:

    export interface InfoRequest extends Request {
      // command: 'info';
      arguments: InfoRequestArguments;
    }

    /** Arguments for 'info' request. */
    export interface InfoRequestArguments {
      /** The thread ID to retrieve info from */
      threadId: number;

      /** The name of the object to retrieve info about. Valid options are:
       *
       * <empty string> - Returns info for the current function or method the current thread is stopped at.
       * className - Returns the declaration of the specified class
       * functionName - Returns the declaration of the specified function
       * cls::method - Returns the declaration of the specified class method
       * cls::constant - Returns the declaration of the specified class constant
       * cls:$prop - Returns the declaration of the specified class property
       */
      object: string;
    }

    /** Response for 'info' request. */
    export interface InfoResponse extends Response {
      body: {
        /** The requested declaration information. */
        info: string;
      };
    }

Reviewed By: velocityboy

Differential Revision: D9780914

fbshipit-source-id: f4dfc4307a9a34421ec6e7f90f30c93675b2cacb