options: added basic SASL XOAUTH2 support

Added the ability to specify an XOAUTH2 bearer token [RFC6750] via the
option CURLOPT_XOAUTH2_BEARER for authentication using RFC6749 "OAuth
2.0 Authorization Framework".

lib/urldata.h

@@ -876,6 +876,8 @@ struct connectdata {
   char *passwd;  /* password string, allocated */
   char *options; /* options string, allocated */
 
+  char *xoauth2_bearer; /* bearer token for xoauth2, allocated */
+
   char *proxyuser;    /* proxy user name string, allocated */
   char *proxypasswd;  /* proxy password string, allocated */
   curl_proxytype proxytype; /* what kind of proxy that is in use */
@@ -1367,6 +1369,8 @@ enum dupstring {
   STRING_TLSAUTH_PASSWORD,     /* TLS auth <password> */
 #endif
 
+  STRING_BEARER,          /* <bearer>, if used */
+
   /* -- end of strings -- */
   STRING_LAST /* not used, just an end-of-list marker */
 };