composer dependency on spid-php-lib, update & include vendor

this pulls in the spid-php-lib from a temporary side-branch until italia/spid-php-lib#66 gets merged
it is self-documenting on which version of spid-php-lib we're bundling (just look in composer.json)
the supplied bundle.sh script can be used by the maintainer to update the bundled copy of spid-php-lib in an automated way, guaranteeing it is a 1-to-1 copy of the original
for an authentication-related plugin like this one, we need to make the build reproducible and to self-document all files not written by us which land into the plugin

bundle.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+# This script can be used to make it easier to upgrade the spid-php-lib dependency of this plugin
+# Run it from the root of the project
+
+set -e
+rm -rf spid-php-lib
+composer install --no-dev
+rsync -rvx vendor/italia/spid-php-lib .
+cd spid-php-lib
+composer install --no-dev

composer.json

@@ -7,8 +7,15 @@
         "name": "Paolo Greppi",
         "email": "[email protected]"
     }],
+    "repositories": [
+        {
+            "type": "vcs",
+            "url": "https://github.com/italia/spid-php-lib"
+        }
+    ],
     "require": {
-
+        "php": ">=7.0",
+        "italia/spid-php-lib": "dev-feature/issue-65-certs#0b58ecd32b4909ff207f2a47b3dcac8b7507b2fb as 0.30.0"
     },
     "require-dev": {
         "squizlabs/php_codesniffer": "*",
@@ -35,4 +42,4 @@
             "phpcbf": "Fix coding standards warnings/errors automatically with PHP Code Beautifier"
         }
     }
-}
+}