The IP address or DNS name of the proxy to be used.
If used, this must be non-NULL and be a valid DNS name. The name must not exceed TOX_MAX_HOSTNAME_LENGTH characters, and be in a NUL- terminated C string format (TOX_MAX_HOSTNAME_LENGTH includes the NUL byte).
This member is ignored (it can be NULL) if proxy_type is ProxyType.NONE.
The data pointed at by this member is owned by the user, so must outlive the options object.