JwtUtils: handle DER <-> raw signature conversion in-house, drop fgrosse/phpasn1 dependency

normally I would hesitate to reinvent the wheel, but we only need a tiny subset of the ASN.1 spec which is trivial to implement by itself.
I'd rather this than depend on another library that could introduce security vulnerabilities (I'm looking at you, jsonmapper).

closes #5935

composer.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "74166e6c2f09b356c83a951efef349f2",
+    "content-hash": "fa059375785ed5b842af30c6b99c572f",
     "packages": [
         {
             "name": "adhocore/json-comment",
@@ -120,82 +120,6 @@
             ],
             "time": "2023-01-15T23:15:59+00:00"
         },
-        {
-            "name": "fgrosse/phpasn1",
-            "version": "v2.5.0",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/fgrosse/PHPASN1.git",
-                "reference": "42060ed45344789fb9f21f9f1864fc47b9e3507b"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/fgrosse/PHPASN1/zipball/42060ed45344789fb9f21f9f1864fc47b9e3507b",
-                "reference": "42060ed45344789fb9f21f9f1864fc47b9e3507b",
-                "shasum": ""
-            },
-            "require": {
-                "php": "^7.1 || ^8.0"
-            },
-            "require-dev": {
-                "php-coveralls/php-coveralls": "~2.0",
-                "phpunit/phpunit": "^7.0 || ^8.0 || ^9.0"
-            },
-            "suggest": {
-                "ext-bcmath": "BCmath is the fallback extension for big integer calculations",
-                "ext-curl": "For loading OID information from the web if they have not bee defined statically",
-                "ext-gmp": "GMP is the preferred extension for big integer calculations",
-                "phpseclib/bcmath_compat": "BCmath polyfill for servers where neither GMP nor BCmath is available"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "2.0.x-dev"
-                }
-            },
-            "autoload": {
-                "psr-4": {
-                    "FG\\": "lib/"
-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "Friedrich Große",
-                    "email": "friedrich.grosse@gmail.com",
-                    "homepage": "https://github.com/FGrosse",
-                    "role": "Author"
-                },
-                {
-                    "name": "All contributors",
-                    "homepage": "https://github.com/FGrosse/PHPASN1/contributors"
-                }
-            ],
-            "description": "A PHP Framework that allows you to encode and decode arbitrary ASN.1 structures using the ITU-T X.690 Encoding Rules.",
-            "homepage": "https://github.com/FGrosse/PHPASN1",
-            "keywords": [
-                "DER",
-                "asn.1",
-                "asn1",
-                "ber",
-                "binary",
-                "decoding",
-                "encoding",
-                "x.509",
-                "x.690",
-                "x509",
-                "x690"
-            ],
-            "support": {
-                "issues": "https://github.com/fgrosse/PHPASN1/issues",
-                "source": "https://github.com/fgrosse/PHPASN1/tree/v2.5.0"
-            },
-            "abandoned": true,
-            "time": "2022-12-19T11:08:26+00:00"
-        },
         {
             "name": "pocketmine/bedrock-block-upgrade-schema",
             "version": "3.1.0",