sql: alias SET client_encoding to SET NAMES

`SET NAMES` is the standard SQL form to indicate which encoding the
client uses to specify identifiers/strings to the server.

This patch adds support for this syntax and aliases it to `SET
client_encoding`. `RESET NAMES` and `SHOW NAMES` are similarly
supported.

pkg/sql/parser/parse_test.go

@@ -944,6 +944,17 @@ func TestParse2(t *testing.T) {
 			`RESTORE DATABASE foo FROM 'bar'`},
 
 		{`SHOW ALL CLUSTER SETTINGS`, `SHOW CLUSTER SETTING all`},
+
+		{`USE foo`, `SET database = 'foo'`},
+
+		{`SET NAMES foo`, `SET client_encoding = 'foo'`},
+		{`SET NAMES 'foo'`, `SET client_encoding = 'foo'`},
+		{`SET NAMES DEFAULT`, `RESET client_encoding`},
+		{`SET NAMES`, `RESET client_encoding`},
+
+		{`SHOW NAMES`, `SHOW client_encoding`},
+
+		{`RESET NAMES`, `RESET client_encoding`},
 	}
 	for _, d := range testData {
 		stmts, err := Parse(d.sql)