Send a channel message

typedef struct vx_req_session_send_message{    /**    * The common properties for all requests    */    vx_req_base_t base;    /**    * Handle returned from successful vx_req_sessiongroup_add_session request    */    VX_HANDLE session_handle;    /**    * HTTP Content type of the message.    * If an empty string is passed in, this field will default to "text/plain".    * This field will be validated, so it must be in a valid format.    * For a full description of valid content types, see RFC 2046 (https://www.ietf.org/rfc/rfc2046.txt).    */    char *message_header;    /**    * The contents of the message.    * \note The message body must not exceed the length specified by the server in its pre-login configuration.    * The default max message length is 320 bytes (320 characters UTF-8).    */    char *message_body;    /**    * The language (default null, implies "en").    */    char *language;    /**    * Namespace of the custom application stanza (optional, default null).    */    char *application_stanza_namespace;    /**    * Custom application stanza body (optional, default null).    */    char *application_stanza_body;}vx_req_session_send_message_t;