database/sql: allow OpenConnector in a driver.Driver interface

While driver.Connector was previously added to allow non-string
connection arguments and access to the context, most users of
the sql package will continue to rely on a string DSN.

Allow drivers to implement a string DSN to Connector interface
that both allows a single parsing of the string DSN and uses
the Connector interface which passes available context to
the driver dialer.

Fixes #22713

Change-Id: Ia0b862262f4c4670effe2538d0d6d43733fea18d
Reviewed-on: https://go-review.googlesource.com/77550
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/database/sql/sql_test.go

@@ -3523,6 +3523,19 @@ func TestNamedValueCheckerSkip(t *testing.T) {
 	}
 }
 
+func TestOpenConnector(t *testing.T) {
+	Register("testctx", &fakeDriverCtx{})
+	db, err := Open("testctx", "people")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	if _, is := db.connector.(*fakeConnector); !is {
+		t.Fatal("not using *fakeConnector")
+	}
+}
+
 type ctxOnlyDriver struct {
 	fakeDriver
 }