feat: implement user_auth_method_name

This function is used to determine the authentication method when
authenticating a postgres client.

part of picodata/picodata/sbroad!292

NO_DOC=exports
NO_CHANGELOG=exports
NO_TEST=exports

extra/exports

@@ -640,3 +640,4 @@ coio_write_timeout
 
 fiber_set_name_n
 authenticate
+user_auth_method_name

src/CMakeLists.txt

@@ -231,6 +231,7 @@ set(api_headers
     ${PROJECT_SOURCE_DIR}/src/box/iterator_type.h
     ${PROJECT_SOURCE_DIR}/src/box/error.h
     ${PROJECT_SOURCE_DIR}/src/box/lua/tuple.h
+    ${PROJECT_SOURCE_DIR}/src/box/user.h
     ${PROJECT_SOURCE_DIR}/src/lib/core/latch.h
     ${PROJECT_SOURCE_DIR}/src/lib/core/clock.h
     ${PROJECT_SOURCE_DIR}/src/box/decimal.h

src/box/user.cc

@@ -39,6 +39,7 @@
 #include "scoped_guard.h"
 #include "sequence.h"
 #include "tt_static.h"
+#include "authentication.h"
 
 struct universe universe;
 static struct user users[BOX_USER_MAX];
@@ -396,6 +397,14 @@ user_reload_privs(struct user *user)
 	return 0;
 }
 
+const char *
+user_auth_method_name(const char *user_name, uint32_t user_name_len)
+{
+	struct user *user = user_find_by_name(user_name, user_name_len);
+	struct authenticator *auth = user != NULL ? user->def->auth : NULL;
+	return auth != NULL ? auth->method->name : NULL;
+}
+
 /** }}} */
 
 /* {{{ authentication tokens */

src/box/user.h

@@ -143,6 +143,24 @@ credentials_reset(struct credentials *cr, struct user *new_user)
 	credentials_create(cr, new_user);
 }
 
+/** \cond public */
+
+/**
+ * Get an auth method name used by a user.
+ * Returns method name on success and NULL on error.
+ *
+ * This function is used to determine the authentication method when
+ * authenticating a postgres client. There is an another way to determine auth
+ * method name: export user_find_by_name and extract the name from the name
+ * field of the authenticator contained in the user, but this approach seems to
+ * be more complicated, since we still need to export something, as well as
+ * add definition of user and its internal fields in module.h.
+ */
+const char *
+user_auth_method_name(const char *name, uint32_t name_len);
+
+/** \endcond public */
+
 /**
  * For best performance, all users are maintained in this array.
  * Position in the array is store in user->auth_token and also