feature: add Microsoft SQL Server database support

can be used via the pdo_dblib or pdo_odbc extensions.
requires Microsoft SQL Server 2016 or later.

* add schema/migrations
* add database provider
* add setup check for pdo module needed by selected driver (dblib/odbc)
* add default config doc

app/check_setup.php

@@ -23,6 +23,14 @@ if (DB_DRIVER === 'postgres' && !extension_loaded('pdo_pgsql')) {
     throw new Exception('PHP extension required: "pdo_pgsql"');
 }
 
+if (DB_DRIVER === 'odbc' && !extension_loaded('pdo_odbc')) {
+    throw new Exception('PHP extension required: "pdo_odbc"');
+}
+
+if (DB_DRIVER === 'dblib' && !extension_loaded('pdo_dblib')) {
+    throw new Exception('PHP extension required: "pdo_dblib"');
+}
+
 // Check other extensions
 foreach (array('gd', 'mbstring', 'hash', 'openssl', 'json', 'hash', 'ctype', 'filter', 'session', 'dom', 'filter', 'SimpleXML', 'xml') as $ext) {
     if (!extension_loaded($ext)) {