Unify the url of submodule to https url (#4566)

close #4565

.gitmodules

@@ -31,13 +31,13 @@
 	branch = master
 [submodule "contrib/client-c"]
 	path = contrib/client-c
-	url = git@github.com:tikv/client-c.git
+	url = https://github.com/tikv/client-c.git
 [submodule "contrib/tiflash-proxy"]
 	path = contrib/tiflash-proxy
-	url = git@github.com:pingcap/tidb-engine-ext.git
+	url = https://github.com/pingcap/tidb-engine-ext.git
 [submodule "contrib/prometheus-cpp"]
 	path = contrib/prometheus-cpp
-	url = git@github.com:jupp0r/prometheus-cpp.git
+	url = https://github.com/jupp0r/prometheus-cpp.git
 [submodule "contrib/junction"]
 	path = contrib/junction
 	url = https://github.com/preshing/junction.git