sql: support backslash_quote session setting

This setting is based on an previous issue: cockroachdb#45774. This commit set a placeholder for PostgreSQL configuration setting called (backslash_quote)[https://www.postgresql.org/docs/12/runtime-config-compatible.html#GUC-BACKSLASH-QUOTE], and ensured it is compatible with ORMS.

When users try setting the backslash_quote session variable, CRDB will let it pass, but not making any changes. We leave the functionality implementation to future works.

Details:
- Added `backslash_quote` at varGen at `sql/vars.go` without implementing the true functionality

- Tested it with `subtest backslash_quote_test` at `pkg/sql/logictest/testdata/logic_test/set`

- Removed `backslash_quote` from `unsupported_vars.go`

pkg/sql/logictest/testdata/logic_test/set

@@ -476,3 +476,20 @@ SET standard_conforming_strings='true'
 
 statement ok
 SET standard_conforming_strings='on'
+
+
+subtest backslash_quote_test
+
+statement ok
+SET backslash_quote = 'on';
+
+statement ok
+SET backslash_quote = 'off';
+
+statement ok
+SET backslash_quote = 'safe_encoding';
+
+statement error invalid value for parameter "backslash_quote"
+SET backslash_quote = '123';
+
+

pkg/sql/unsupported_vars.go

@@ -64,7 +64,7 @@ var UnsupportedVars = func(ss ...string) map[string]struct{} {
 	// "application_name",
 	"array_nulls",
 	"backend_flush_after",
-	"backslash_quote",
+	//"backslash_quote",
 	// "bytea_output",
 	"check_function_bodies",
 	// "client_encoding",

pkg/sql/vars.go

@@ -831,8 +831,11 @@ var varGen = map[string]sessionVar{
 	// See https://www.postgresql.org/docs/10/static/runtime-config-client.html#GUC-LOC-TIMEOUT
 	`lock_timeout`: makeCompatIntVar(`lock_timeout`, 0),
 
+	// See https://www.postgresql.org/docs/9.1/runtime-config-compatible.html
+	`backslash_quote`: makeCompatStringVar(`backslash_quote`, `safe_encoding`, `on`, `off`, `safe_encoding`),
+
 	// Supported for PG compatibility only.
-	// See https://www.postgresql.org/docs/10/static/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS
+	// See https://www.postgresql.org/docs/10/static/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIER
 	`max_identifier_length`: {
 		Get: func(evalCtx *extendedEvalContext) string { return "128" },
 	},